SQLAlchemy 2.0 Documentation
Frequently Asked Questions
- Installation
- Connections / Engines
- MetaData / Schema¶
- My program is hanging when I say
table.drop()
/metadata.drop_all()
- Does SQLAlchemy support ALTER TABLE, CREATE VIEW, CREATE TRIGGER, Schema Upgrade Functionality?
- How can I sort Table objects in order of their dependency?
- How can I get the CREATE TABLE/ DROP TABLE output as a string?
- How can I subclass Table/Column to provide certain behaviors/configurations?
- My program is hanging when I say
- SQL Expressions
- ORM Configuration
- Performance
- Sessions / Queries
- Third Party Integration Issues
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- MetaData / Schema
- My program is hanging when I say
table.drop()
/metadata.drop_all()
- Does SQLAlchemy support ALTER TABLE, CREATE VIEW, CREATE TRIGGER, Schema Upgrade Functionality?
- How can I sort Table objects in order of their dependency?
- How can I get the CREATE TABLE/ DROP TABLE output as a string?
- How can I subclass Table/Column to provide certain behaviors/configurations?
- My program is hanging when I say
MetaData / Schema¶
My program is hanging when I say table.drop()
/ metadata.drop_all()
¶
This usually corresponds to two conditions: 1. using PostgreSQL, which is really strict about table locks, and 2. you have a connection still open which contains locks on the table and is distinct from the connection being used for the DROP statement. Heres the most minimal version of the pattern:
connection = engine.connect()
result = connection.execute(mytable.select())
mytable.drop(engine)
Above, a connection pool connection is still checked out; furthermore, the result object above also maintains a link to this connection. If “implicit execution” is used, the result will hold this connection opened until the result object is closed or all rows are exhausted.
The call to mytable.drop(engine)
attempts to emit DROP TABLE on a second
connection procured from the Engine
which will lock.
The solution is to close out all connections before emitting DROP TABLE:
connection = engine.connect()
result = connection.execute(mytable.select())
# fully read result sets
result.fetchall()
# close connections
connection.close()
# now locks are removed
mytable.drop(engine)
Does SQLAlchemy support ALTER TABLE, CREATE VIEW, CREATE TRIGGER, Schema Upgrade Functionality?¶
General ALTER support isn’t present in SQLAlchemy directly. For special DDL
on an ad-hoc basis, the DDL
and related constructs can be used.
See Customizing DDL for a discussion on this subject.
A more comprehensive option is to use schema migration tools, such as Alembic or SQLAlchemy-Migrate; see Altering Database Objects through Migrations for discussion on this.
How can I sort Table objects in order of their dependency?¶
This is available via the MetaData.sorted_tables
function:
metadata_obj = MetaData()
# ... add Table objects to metadata
ti = metadata_obj.sorted_tables
for t in ti:
print(t)
How can I get the CREATE TABLE/ DROP TABLE output as a string?¶
Modern SQLAlchemy has clause constructs which represent DDL operations. These can be rendered to strings like any other SQL expression:
from sqlalchemy.schema import CreateTable
print(CreateTable(mytable))
To get the string specific to a certain engine:
print(CreateTable(mytable).compile(engine))
There’s also a special form of Engine
available via
create_mock_engine()
that allows one to dump an entire
metadata creation sequence as a string, using this recipe:
from sqlalchemy import create_mock_engine
def dump(sql, *multiparams, **params):
print(sql.compile(dialect=engine.dialect))
engine = create_mock_engine("postgresql+psycopg2://", dump)
metadata_obj.create_all(engine, checkfirst=False)
The Alembic tool also supports an “offline” SQL generation mode that renders database migrations as SQL scripts.
How can I subclass Table/Column to provide certain behaviors/configurations?¶
Table
and Column
are not good targets for direct subclassing.
However, there are simple ways to get on-construction behaviors using creation
functions, and behaviors related to the linkages between schema objects such as
constraint conventions or naming conventions using attachment events.
An example of many of these
techniques can be seen at Naming Conventions.
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