SQLAlchemy 2.1 Documentation
SQLAlchemy ORM
- ORM Quick Start
- ORM Mapped Class Configuration
- ORM Mapped Class Overview
- Mapping Classes with Declarative
- Integration with dataclasses and attrs¶
- SQL Expressions as Mapped Attributes
- Changing Attribute Behavior
- Composite Column Types
- Mapping Class Inheritance Hierarchies
- Non-Traditional Mappings
- Configuring a Version Counter
- Class Mapping API
- Mapping SQL Expressions
- Relationship Configuration
- ORM Querying Guide
- Using the Session
- Events and Internals
- ORM Extensions
- ORM Examples
Project Versions
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Integration with dataclasses and attrs¶
SQLAlchemy as of version 2.0 features “native dataclass” integration where an Annotated Declarative Table mapping may be turned into a Python dataclass by adding a single mixin or decorator to mapped classes.
New in version 2.0: Integrated dataclass creation with ORM Declarative classes
There are also patterns available that allow existing dataclasses to be mapped, as well as to map classes instrumented by the attrs third party integration library.
Declarative Dataclass Mapping¶
SQLAlchemy Annotated Declarative Table
mappings may be augmented with an additional
mixin class or decorator directive, which will add an additional step to
the Declarative process after the mapping is complete that will convert
the mapped class in-place into a Python dataclass, before completing
the mapping process which applies ORM-specific instrumentation
to the class. The most prominent behavioral addition this provides is
generation of an __init__()
method with fine-grained control over
positional and keyword arguments with or without defaults, as well as
generation of methods like __repr__()
and __eq__()
.
From a PEP 484 typing perspective, the class is recognized
as having Dataclass-specific behaviors, most notably by taking advantage of PEP 681
“Dataclass Transforms”, which allows typing tools to consider the class
as though it were explicitly decorated using the @dataclasses.dataclass
decorator.
Note
Support for PEP 681 in typing tools as of April 4, 2023 is limited and is currently known to be supported by Pyright as well as Mypy as of version 1.2. Note that Mypy 1.1.1 introduced PEP 681 support but did not correctly accommodate Python descriptors which will lead to errors when using SQLAlchemy’s ORM mapping scheme.
See also
https://peps.python.org/pep-0681/#the-dataclass-transform-decorator - background on how libraries like SQLAlchemy enable PEP 681 support
Dataclass conversion may be added to any Declarative class either by adding the
MappedAsDataclass
mixin to a DeclarativeBase
class
hierarchy, or for decorator mapping by using the
registry.mapped_as_dataclass()
class decorator.
The MappedAsDataclass
mixin may be applied either
to the Declarative Base
class or any superclass, as in the example
below:
from sqlalchemy.orm import DeclarativeBase
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import MappedAsDataclass
class Base(MappedAsDataclass, DeclarativeBase):
"""subclasses will be converted to dataclasses"""
class User(Base):
__tablename__ = "user_account"
id: Mapped[int] = mapped_column(init=False, primary_key=True)
name: Mapped[str]
Or may be applied directly to classes that extend from the Declarative base:
from sqlalchemy.orm import DeclarativeBase
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import MappedAsDataclass
class Base(DeclarativeBase):
pass
class User(MappedAsDataclass, Base):
"""User class will be converted to a dataclass"""
__tablename__ = "user_account"
id: Mapped[int] = mapped_column(init=False, primary_key=True)
name: Mapped[str]
When using the decorator form, only the registry.mapped_as_dataclass()
decorator is supported:
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import registry
reg = registry()
@reg.mapped_as_dataclass
class User:
__tablename__ = "user_account"
id: Mapped[int] = mapped_column(init=False, primary_key=True)
name: Mapped[str]
Class level feature configuration¶
Support for dataclasses features is partial. Currently supported are
the init
, repr
, eq
, order
and unsafe_hash
features,
match_args
and kw_only
are supported on Python 3.10+.
Currently not supported are the frozen
and slots
features.
When using the mixin class form with MappedAsDataclass
,
class configuration arguments are passed as class-level parameters:
from sqlalchemy.orm import DeclarativeBase
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import MappedAsDataclass
class Base(DeclarativeBase):
pass
class User(MappedAsDataclass, Base, repr=False, unsafe_hash=True):
"""User class will be converted to a dataclass"""
__tablename__ = "user_account"
id: Mapped[int] = mapped_column(init=False, primary_key=True)
name: Mapped[str]
When using the decorator form with registry.mapped_as_dataclass()
,
class configuration arguments are passed to the decorator directly:
from sqlalchemy.orm import registry
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
reg = registry()
@reg.mapped_as_dataclass(unsafe_hash=True)
class User:
"""User class will be converted to a dataclass"""
__tablename__ = "user_account"
id: Mapped[int] = mapped_column(init=False, primary_key=True)
name: Mapped[str]
For background on dataclass class options, see the dataclasses documentation at @dataclasses.dataclass.
Attribute Configuration¶
SQLAlchemy native dataclasses differ from normal dataclasses in that
attributes to be mapped are described using the Mapped
generic annotation container in all cases. Mappings follow the same
forms as those documented at Declarative Table with mapped_column(), and all
features of mapped_column()
and Mapped
are supported.
Additionally, ORM attribute configuration constructs including
mapped_column()
, relationship()
and composite()
support per-attribute field options, including init
, default
,
default_factory
and repr
. The names of these arguments is fixed
as specified in PEP 681. Functionality is equivalent to dataclasses:
init
, as inmapped_column.init
,relationship.init
, if False indicates the field should not be part of the__init__()
methoddefault
, as inmapped_column.default
,relationship.default
indicates a default value for the field as given as a keyword argument in the__init__()
method.default_factory
, as inmapped_column.default_factory
,relationship.default_factory
, indicates a callable function that will be invoked to generate a new default value for a parameter if not passed explicitly to the__init__()
method.repr
True by default, indicates the field should be part of the generated__repr__()
method
Another key difference from dataclasses is that default values for attributes
must be configured using the default
parameter of the ORM construct,
such as mapped_column(default=None)
. A syntax that resembles dataclass
syntax which accepts simple Python values as defaults without using
@dataclases.field()
is not supported.
As an example using mapped_column()
, the mapping below will
produce an __init__()
method that accepts only the fields name
and
fullname
, where name
is required and may be passed positionally,
and fullname
is optional. The id
field, which we expect to be
database-generated, is not part of the constructor at all:
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import registry
reg = registry()
@reg.mapped_as_dataclass
class User:
__tablename__ = "user_account"
id: Mapped[int] = mapped_column(init=False, primary_key=True)
name: Mapped[str]
fullname: Mapped[str] = mapped_column(default=None)
# 'fullname' is optional keyword argument
u1 = User("name")
Column Defaults¶
In order to accommodate the name overlap of the default
argument with
the existing Column.default
parameter of the Column
construct, the mapped_column()
construct disambiguates the two
names by adding a new parameter mapped_column.insert_default
,
which will be populated directly into the
Column.default
parameter of Column
,
independently of what may be set on
mapped_column.default
, which is always used for the
dataclasses configuration. For example, to configure a datetime column with
a Column.default
set to the func.utc_timestamp()
SQL function,
but where the parameter is optional in the constructor:
from datetime import datetime
from sqlalchemy import func
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import registry
reg = registry()
@reg.mapped_as_dataclass
class User:
__tablename__ = "user_account"
id: Mapped[int] = mapped_column(init=False, primary_key=True)
created_at: Mapped[datetime] = mapped_column(
insert_default=func.utc_timestamp(), default=None
)
With the above mapping, an INSERT
for a new User
object where no
parameter for created_at
were passed proceeds as:
>>> with Session(e) as session:
... session.add(User())
... session.commit()
BEGIN (implicit)
INSERT INTO user_account (created_at) VALUES (utc_timestamp())
[generated in 0.00010s] ()
COMMIT
Integration with Annotated¶
The approach introduced at Mapping Whole Column Declarations to Python Types illustrates
how to use PEP 593 Annotated
objects to package whole
mapped_column()
constructs for re-use. This feature is supported
with the dataclasses feature. One aspect of the feature however requires
a workaround when working with typing tools, which is that the
PEP 681-specific arguments init
, default
, repr
, and default_factory
must be on the right hand side, packaged into an explicit mapped_column()
construct, in order for the typing tool to interpret the attribute correctly.
As an example, the approach below will work perfectly fine at runtime,
however typing tools will consider the User()
construction to be
invalid, as they do not see the init=False
parameter present:
from typing import Annotated
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import registry
# typing tools will ignore init=False here
intpk = Annotated[int, mapped_column(init=False, primary_key=True)]
reg = registry()
@reg.mapped_as_dataclass
class User:
__tablename__ = "user_account"
id: Mapped[intpk]
# typing error: Argument missing for parameter "id"
u1 = User()
Instead, mapped_column()
must be present on the right side
as well with an explicit setting for mapped_column.init
;
the other arguments can remain within the Annotated
construct:
from typing import Annotated
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import registry
intpk = Annotated[int, mapped_column(primary_key=True)]
reg = registry()
@reg.mapped_as_dataclass
class User:
__tablename__ = "user_account"
# init=False and other pep-681 arguments must be inline
id: Mapped[intpk] = mapped_column(init=False)
u1 = User()
Using mixins and abstract superclasses¶
Any mixins or base classes that are used in a MappedAsDataclass
mapped class which include Mapped
attributes must themselves be
part of a MappedAsDataclass
hierarchy, such as in the example below using a mixin:
class Mixin(MappedAsDataclass):
create_user: Mapped[int] = mapped_column()
update_user: Mapped[Optional[int]] = mapped_column(default=None, init=False)
class Base(DeclarativeBase, MappedAsDataclass):
pass
class User(Base, Mixin):
__tablename__ = "sys_user"
uid: Mapped[str] = mapped_column(
String(50), init=False, default_factory=uuid4, primary_key=True
)
username: Mapped[str] = mapped_column()
email: Mapped[str] = mapped_column()
Python type checkers which support PEP 681 will otherwise not consider attributes from non-dataclass mixins to be part of the dataclass.
Deprecated since version 2.0.8: Using mixins and abstract bases within
MappedAsDataclass
or
registry.mapped_as_dataclass()
hierarchies which are not
themselves dataclasses is deprecated, as these fields are not supported
by PEP 681 as belonging to the dataclass. A warning is emitted for this
case which will later be an error.
See also
When transforming <cls> to a dataclass, attribute(s) originate from superclass <cls> which is not a dataclass. - background on rationale
Relationship Configuration¶
The Mapped
annotation in combination with
relationship()
is used in the same way as described at
Basic Relationship Patterns. When specifying a collection-based
relationship()
as an optional keyword argument, the
relationship.default_factory
parameter must be passed and it
must refer to the collection class that’s to be used. Many-to-one and
scalar object references may make use of
relationship.default
if the default value is to be None
:
from typing import List
from sqlalchemy import ForeignKey
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import registry
from sqlalchemy.orm import relationship
reg = registry()
@reg.mapped_as_dataclass
class Parent:
__tablename__ = "parent"
id: Mapped[int] = mapped_column(primary_key=True)
children: Mapped[List["Child"]] = relationship(
default_factory=list, back_populates="parent"
)
@reg.mapped_as_dataclass
class Child:
__tablename__ = "child"
id: Mapped[int] = mapped_column(primary_key=True)
parent_id: Mapped[int] = mapped_column(ForeignKey("parent.id"))
parent: Mapped["Parent"] = relationship(default=None)
The above mapping will generate an empty list for Parent.children
when a
new Parent()
object is constructed without passing children
, and
similarly a None
value for Child.parent
when a new Child()
object
is constructed without passing parent
.
While the relationship.default_factory
can be automatically
derived from the given collection class of the relationship()
itself, this would break compatibility with dataclasses, as the presence
of relationship.default_factory
or
relationship.default
is what determines if the parameter is
to be required or optional when rendered into the __init__()
method.
Using Non-Mapped Dataclass Fields¶
When using Declarative dataclasses, non-mapped fields may be used on the
class as well, which will be part of the dataclass construction process but
will not be mapped. Any field that does not use Mapped
will
be ignored by the mapping process. In the example below, the fields
ctrl_one
and ctrl_two
will be part of the instance-level state
of the object, but will not be persisted by the ORM:
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import registry
reg = registry()
@reg.mapped_as_dataclass
class Data:
__tablename__ = "data"
id: Mapped[int] = mapped_column(init=False, primary_key=True)
status: Mapped[str]
ctrl_one: Optional[str] = None
ctrl_two: Optional[str] = None
Instance of Data
above can be created as:
d1 = Data(status="s1", ctrl_one="ctrl1", ctrl_two="ctrl2")
A more real world example might be to make use of the Dataclasses
InitVar
feature in conjunction with the __post_init__()
feature to
receive init-only fields that can be used to compose persisted data.
In the example below, the User
class is declared using id
, name
and password_hash
as mapped features,
but makes use of init-only password
and repeat_password
fields to
represent the user creation process (note: to run this example, replace
the function your_crypt_function_here()
with a third party crypt
function, such as bcrypt or
argon2-cffi):
from dataclasses import InitVar
from typing import Optional
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import registry
reg = registry()
@reg.mapped_as_dataclass
class User:
__tablename__ = "user_account"
id: Mapped[int] = mapped_column(init=False, primary_key=True)
name: Mapped[str]
password: InitVar[str]
repeat_password: InitVar[str]
password_hash: Mapped[str] = mapped_column(init=False, nullable=False)
def __post_init__(self, password: str, repeat_password: str):
if password != repeat_password:
raise ValueError("passwords do not match")
self.password_hash = your_crypt_function_here(password)
The above object is created with parameters password
and
repeat_password
, which are consumed up front so that the password_hash
variable may be generated:
>>> u1 = User(name="some_user", password="xyz", repeat_password="xyz")
>>> u1.password_hash
'$6$9ppc... (example crypted string....)'
Changed in version 2.0.0rc1: When using registry.mapped_as_dataclass()
or MappedAsDataclass
, fields that do not include the
Mapped
annotation may be included, which will be treated as part
of the resulting dataclass but not be mapped, without the need to
also indicate the __allow_unmapped__
class attribute. Previous 2.0
beta releases would require this attribute to be explicitly present,
even though the purpose of this attribute was only to allow legacy
ORM typed mappings to continue to function.
Integrating with Alternate Dataclass Providers such as Pydantic¶
Warning
The dataclass layer of Pydantic is not fully compatible with SQLAlchemy’s class instrumentation without additional internal changes, and many features such as related collections may not work correctly.
For Pydantic compatibility, please consider the SQLModel ORM which is built with Pydantic on top of SQLAlchemy ORM, which includes special implementation details which explicitly resolve these incompatibilities.
SQLAlchemy’s MappedAsDataclass
class
and registry.mapped_as_dataclass()
method call directly into
the Python standard library dataclasses.dataclass
class decorator, after
the declarative mapping process has been applied to the class. This
function call may be swapped out for alternateive dataclasses providers,
such as that of Pydantic, using the dataclass_callable
parameter
accepted by MappedAsDataclass
as a class keyword argument
as well as by registry.mapped_as_dataclass()
:
from sqlalchemy.orm import DeclarativeBase
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import MappedAsDataclass
from sqlalchemy.orm import registry
class Base(
MappedAsDataclass,
DeclarativeBase,
dataclass_callable=pydantic.dataclasses.dataclass,
):
pass
class User(Base):
__tablename__ = "user"
id: Mapped[int] = mapped_column(primary_key=True)
name: Mapped[str]
The above User
class will be applied as a dataclass, using Pydantic’s
pydantic.dataclasses.dataclasses
callable. The process is available
both for mapped classes as well as mixins that extend from
MappedAsDataclass
or which have
registry.mapped_as_dataclass()
applied directly.
New in version 2.0.4: Added the dataclass_callable
class and method
parameters for MappedAsDataclass
and
registry.mapped_as_dataclass()
, and adjusted some of the
dataclass internals to accommodate more strict dataclass functions such as
that of Pydantic.
Applying ORM Mappings to an existing dataclass (legacy dataclass use)¶
Legacy Feature
The approaches described here are superseded by the Declarative Dataclass Mapping feature new in the 2.0 series of SQLAlchemy. This newer version of the feature builds upon the dataclass support first added in version 1.4, which is described in this section.
To map an existing dataclass, SQLAlchemy’s “inline” declarative directives cannot be used directly; ORM directives are assigned using one of three techniques:
Using “Declarative with Imperative Table”, the table / column to be mapped is defined using a
Table
object assigned to the__table__
attribute of the class; relationships are defined within__mapper_args__
dictionary. The class is mapped using theregistry.mapped()
decorator. An example is below at Mapping pre-existing dataclasses using Declarative With Imperative Table.Using full “Declarative”, the Declarative-interpreted directives such as
Column
,relationship()
are added to the.metadata
dictionary of thedataclasses.field()
construct, where they are consumed by the declarative process. The class is again mapped using theregistry.mapped()
decorator. See the example below at Mapping pre-existing dataclasses using Declarative-style fields.An “Imperative” mapping can be applied to an existing dataclass using the
registry.map_imperatively()
method to produce the mapping in exactly the same way as described at Imperative Mapping. This is illustrated below at Mapping pre-existing dataclasses using Imperative Mapping.
The general process by which SQLAlchemy applies mappings to a dataclass
is the same as that of an ordinary class, but also includes that
SQLAlchemy will detect class-level attributes that were part of the
dataclasses declaration process and replace them at runtime with
the usual SQLAlchemy ORM mapped attributes. The __init__
method that
would have been generated by dataclasses is left intact, as is the same
for all the other methods that dataclasses generates such as
__eq__()
, __repr__()
, etc.
Mapping pre-existing dataclasses using Declarative With Imperative Table¶
An example of a mapping using @dataclass
using
Declarative with Imperative Table (a.k.a. Hybrid Declarative) is below. A complete
Table
object is constructed explicitly and assigned to the
__table__
attribute. Instance fields are defined using normal dataclass
syntaxes. Additional MapperProperty
definitions such as relationship()
, are placed in the
__mapper_args__ class-level
dictionary underneath the properties
key, corresponding to the
Mapper.properties
parameter:
from __future__ import annotations
from dataclasses import dataclass, field
from typing import List, Optional
from sqlalchemy import Column, ForeignKey, Integer, String, Table
from sqlalchemy.orm import registry, relationship
mapper_registry = registry()
@mapper_registry.mapped
@dataclass
class User:
__table__ = Table(
"user",
mapper_registry.metadata,
Column("id", Integer, primary_key=True),
Column("name", String(50)),
Column("fullname", String(50)),
Column("nickname", String(12)),
)
id: int = field(init=False)
name: Optional[str] = None
fullname: Optional[str] = None
nickname: Optional[str] = None
addresses: List[Address] = field(default_factory=list)
__mapper_args__ = { # type: ignore
"properties": {
"addresses": relationship("Address"),
}
}
@mapper_registry.mapped
@dataclass
class Address:
__table__ = Table(
"address",
mapper_registry.metadata,
Column("id", Integer, primary_key=True),
Column("user_id", Integer, ForeignKey("user.id")),
Column("email_address", String(50)),
)
id: int = field(init=False)
user_id: int = field(init=False)
email_address: Optional[str] = None
In the above example, the User.id
, Address.id
, and Address.user_id
attributes are defined as field(init=False)
. This means that parameters for
these won’t be added to __init__()
methods, but
Session
will still be able to set them after getting their values
during flush from autoincrement or other default value generator. To
allow them to be specified in the constructor explicitly, they would instead
be given a default value of None
.
For a relationship()
to be declared separately, it needs to be
specified directly within the Mapper.properties
dictionary
which itself is specified within the __mapper_args__
dictionary, so that it
is passed to the constructor for Mapper
. An alternative to this
approach is in the next example.
Warning
Declaring a dataclass field()
setting a default
together with init=False
will not work as would be expected with a totally plain dataclass,
since the SQLAlchemy class instrumentation will replace
the default value set on the class by the dataclass creation process.
Use default_factory
instead. This adaptation is done automatically when
making use of Declarative Dataclass Mapping.
Mapping pre-existing dataclasses using Declarative-style fields¶
Legacy Feature
This approach to Declarative mapping with dataclasses should be considered as legacy. It will remain supported however is unlikely to offer any advantages against the new approach detailed at Declarative Dataclass Mapping.
Note that mapped_column() is not supported with this use;
the Column
construct should continue to be used to declare
table metadata within the metadata
field of dataclasses.field()
.
The fully declarative approach requires that Column
objects
are declared as class attributes, which when using dataclasses would conflict
with the dataclass-level attributes. An approach to combine these together
is to make use of the metadata
attribute on the dataclass.field
object, where SQLAlchemy-specific mapping information may be supplied.
Declarative supports extraction of these parameters when the class
specifies the attribute __sa_dataclass_metadata_key__
. This also
provides a more succinct method of indicating the relationship()
association:
from __future__ import annotations
from dataclasses import dataclass, field
from typing import List
from sqlalchemy import Column, ForeignKey, Integer, String
from sqlalchemy.orm import registry, relationship
mapper_registry = registry()
@mapper_registry.mapped
@dataclass
class User:
__tablename__ = "user"
__sa_dataclass_metadata_key__ = "sa"
id: int = field(init=False, metadata={"sa": Column(Integer, primary_key=True)})
name: str = field(default=None, metadata={"sa": Column(String(50))})
fullname: str = field(default=None, metadata={"sa": Column(String(50))})
nickname: str = field(default=None, metadata={"sa": Column(String(12))})
addresses: List[Address] = field(
default_factory=list, metadata={"sa": relationship("Address")}
)
@mapper_registry.mapped
@dataclass
class Address:
__tablename__ = "address"
__sa_dataclass_metadata_key__ = "sa"
id: int = field(init=False, metadata={"sa": Column(Integer, primary_key=True)})
user_id: int = field(init=False, metadata={"sa": Column(ForeignKey("user.id"))})
email_address: str = field(default=None, metadata={"sa": Column(String(50))})
Using Declarative Mixins with pre-existing dataclasses¶
In the section Composing Mapped Hierarchies with Mixins, Declarative Mixin classes
are introduced. One requirement of declarative mixins is that certain
constructs that can’t be easily duplicated must be given as callables,
using the declared_attr
decorator, such as in the
example at Mixing in Relationships:
class RefTargetMixin:
@declared_attr
def target_id(cls) -> Mapped[int]:
return mapped_column("target_id", ForeignKey("target.id"))
@declared_attr
def target(cls):
return relationship("Target")
This form is supported within the Dataclasses field()
object by using
a lambda to indicate the SQLAlchemy construct inside the field()
.
Using declared_attr()
to surround the lambda is optional.
If we wanted to produce our User
class above where the ORM fields
came from a mixin that is itself a dataclass, the form would be:
@dataclass
class UserMixin:
__tablename__ = "user"
__sa_dataclass_metadata_key__ = "sa"
id: int = field(init=False, metadata={"sa": Column(Integer, primary_key=True)})
addresses: List[Address] = field(
default_factory=list, metadata={"sa": lambda: relationship("Address")}
)
@dataclass
class AddressMixin:
__tablename__ = "address"
__sa_dataclass_metadata_key__ = "sa"
id: int = field(init=False, metadata={"sa": Column(Integer, primary_key=True)})
user_id: int = field(
init=False, metadata={"sa": lambda: Column(ForeignKey("user.id"))}
)
email_address: str = field(default=None, metadata={"sa": Column(String(50))})
@mapper_registry.mapped
class User(UserMixin):
pass
@mapper_registry.mapped
class Address(AddressMixin):
pass
New in version 1.4.2: Added support for “declared attr” style mixin attributes,
namely relationship()
constructs as well as Column
objects with foreign key declarations, to be used within “Dataclasses
with Declarative Table” style mappings.
Mapping pre-existing dataclasses using Imperative Mapping¶
As described previously, a class which is set up as a dataclass using the
@dataclass
decorator can then be further decorated using the
registry.mapped()
decorator in order to apply declarative-style
mapping to the class. As an alternative to using the
registry.mapped()
decorator, we may also pass the class through the
registry.map_imperatively()
method instead, so that we may pass all
Table
and Mapper
configuration imperatively to
the function rather than having them defined on the class itself as class
variables:
from __future__ import annotations
from dataclasses import dataclass
from dataclasses import field
from typing import List
from sqlalchemy import Column
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import String
from sqlalchemy import Table
from sqlalchemy.orm import registry
from sqlalchemy.orm import relationship
mapper_registry = registry()
@dataclass
class User:
id: int = field(init=False)
name: str = None
fullname: str = None
nickname: str = None
addresses: List[Address] = field(default_factory=list)
@dataclass
class Address:
id: int = field(init=False)
user_id: int = field(init=False)
email_address: str = None
metadata_obj = MetaData()
user = Table(
"user",
metadata_obj,
Column("id", Integer, primary_key=True),
Column("name", String(50)),
Column("fullname", String(50)),
Column("nickname", String(12)),
)
address = Table(
"address",
metadata_obj,
Column("id", Integer, primary_key=True),
Column("user_id", Integer, ForeignKey("user.id")),
Column("email_address", String(50)),
)
mapper_registry.map_imperatively(
User,
user,
properties={
"addresses": relationship(Address, backref="user", order_by=address.c.id),
},
)
mapper_registry.map_imperatively(Address, address)
The same warning mentioned in Mapping pre-existing dataclasses using Declarative With Imperative Table applies when using this mapping style.
Applying ORM mappings to an existing attrs class¶
The attrs library is a popular third party library that provides similar features as dataclasses, with many additional features provided not found in ordinary dataclasses.
A class augmented with attrs uses the @define
decorator. This decorator
initiates a process to scan the class for attributes that define the class’
behavior, which are then used to generate methods, documentation, and
annotations.
The SQLAlchemy ORM supports mapping an attrs class using Declarative with Imperative Table or Imperative mapping. The general form of these two styles is fully equivalent to the Mapping pre-existing dataclasses using Declarative-style fields and Mapping pre-existing dataclasses using Declarative With Imperative Table mapping forms used with dataclasses, where the inline attribute directives used by dataclasses or attrs are unchanged, and SQLAlchemy’s table-oriented instrumentation is applied at runtime.
The @define
decorator of attrs by default replaces the annotated class
with a new __slots__ based class, which is not supported. When using the old
style annotation @attr.s
or using define(slots=False)
, the class
does not get replaced. Furthermore attrs removes its own class-bound attributes
after the decorator runs, so that SQLAlchemy’s mapping process takes over these
attributes without any issue. Both decorators, @attr.s
and @define(slots=False)
work with SQLAlchemy.
Mapping attrs with Declarative “Imperative Table”¶
In the “Declarative with Imperative Table” style, a Table
object is declared inline with the declarative class. The
@define
decorator is applied to the class first, then the
registry.mapped()
decorator second:
from __future__ import annotations
from typing import List
from typing import Optional
from attrs import define
from sqlalchemy import Column
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import String
from sqlalchemy import Table
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import registry
from sqlalchemy.orm import relationship
mapper_registry = registry()
@mapper_registry.mapped
@define(slots=False)
class User:
__table__ = Table(
"user",
mapper_registry.metadata,
Column("id", Integer, primary_key=True),
Column("name", String(50)),
Column("FullName", String(50), key="fullname"),
Column("nickname", String(12)),
)
id: Mapped[int]
name: Mapped[str]
fullname: Mapped[str]
nickname: Mapped[str]
addresses: Mapped[List[Address]]
__mapper_args__ = { # type: ignore
"properties": {
"addresses": relationship("Address"),
}
}
@mapper_registry.mapped
@define(slots=False)
class Address:
__table__ = Table(
"address",
mapper_registry.metadata,
Column("id", Integer, primary_key=True),
Column("user_id", Integer, ForeignKey("user.id")),
Column("email_address", String(50)),
)
id: Mapped[int]
user_id: Mapped[int]
email_address: Mapped[Optional[str]]
Note
The attrs
slots=True
option, which enables __slots__
on
a mapped class, cannot be used with SQLAlchemy mappings without fully
implementing alternative
attribute instrumentation, as mapped
classes normally rely upon direct access to __dict__
for state storage.
Behavior is undefined when this option is present.
Mapping attrs with Imperative Mapping¶
Just as is the case with dataclasses, we can make use of
registry.map_imperatively()
to map an existing attrs
class
as well:
from __future__ import annotations
from typing import List
from attrs import define
from sqlalchemy import Column
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import String
from sqlalchemy import Table
from sqlalchemy.orm import registry
from sqlalchemy.orm import relationship
mapper_registry = registry()
@define(slots=False)
class User:
id: int
name: str
fullname: str
nickname: str
addresses: List[Address]
@define(slots=False)
class Address:
id: int
user_id: int
email_address: Optional[str]
metadata_obj = MetaData()
user = Table(
"user",
metadata_obj,
Column("id", Integer, primary_key=True),
Column("name", String(50)),
Column("fullname", String(50)),
Column("nickname", String(12)),
)
address = Table(
"address",
metadata_obj,
Column("id", Integer, primary_key=True),
Column("user_id", Integer, ForeignKey("user.id")),
Column("email_address", String(50)),
)
mapper_registry.map_imperatively(
User,
user,
properties={
"addresses": relationship(Address, backref="user", order_by=address.c.id),
},
)
mapper_registry.map_imperatively(Address, address)
The above form is equivalent to the previous example using Declarative with Imperative Table.
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