The reason AddConstraint/DropConstraint need the Constraint attached to
the table is because ADD CONSTRAINT / DROP CONSTRAINT in SQL require the
table name, as these are related to ALTER TABLE <tablename>. I
recommend you use Alembic ops to emit ADD CONSTRAINT / DROP CONSTRAINT
without requiring Table logic, see
http://alembic.zzzcomputing.com/en/latest/ops.html#alembic.operations.Operations
.
On 11/18/2016 05:34 PM, Jonathan Rogers wrote:
Yes, the constraints are defined in Python and have names explicitly
defined. The CheckConstraint I need to use is defined in a
@declared_attr method for the column in a base class from which the
mapped class inherits. I did find the CheckConstraint I need in
MappedClass.__table__.c.column_name.constraints, but it is not bound to
a table, so when I pass it to DropConstraint(), I get:
sqlalchemy.exc.InvalidRequestError: This constraint is not bound to a
table. Did you mean to call table.append_constraint(constraint) ?
If I pass the the CheckConstraint to
MappedClass.__table__.append_constraint(), DropConstraint() works, but I
don't understand why I need to explicitly add the constraint to the table.
On Friday, November 18, 2016 at 5:11:10 PM UTC-5, Mike Bayer wrote:
Assuming here your constraints are already defined in Python and we are
not talking about using reflection, we're talking about the CHECK
constraint objects you've already built. If they are defined for a
Table then they would be in table.constraints. If you have them on the
Column then it's true they might only be in column.constraints in that
case. If you're defining them in either place then you'd need to look
in either place. It might be best to go with all table-bound CHECK
constraints as I'm not sure it's as flexible to do add/drop with the
column-level ones. At the very least, constraints need to have an
explicit name in order to support ADD/DROP.
Alembic's docs get into this a bit here:
http://alembic.zzzcomputing.com/en/latest/naming.html
<http://alembic.zzzcomputing.com/en/latest/naming.html> where it talks
about the "naming convention" feature of SQLAlchemy that may or may not
be of use here.
On 11/18/2016 04:43 PM, Jonathan Rogers wrote:
> I have a Declarative-instrumented class with several constraints,
some
> defined at the table level and some on a column. AFAICT, all the
> constraints are configured correctly because they are rendered
correctly
> by CreateTable() when called with the class's Table instance. In
order
> to import some data into my carefully designed model, I need to drop
> some constraints, then recreate them NOT VALID. I want to use the
> existing Table instance to execute SQL rather than typing
duplicate SQL
> manually. However, I'm having trouble introspecting the Table
instance
> to find a specific CheckConstraint defined on a specific column. The
> Table instance's "constraints" property does not contain the
constraint
> I need. I can find a CheckConstraint instance in the column's
> "constraints" property but it's not bound to a Table so I can't just
> pass it to DropConstraint(). What is the proper way to get a
> CheckConstraint instance defined on a column that I can pass to
> DropConstraint() and AddConstraint()?
>
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> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
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> http://www.sqlalchemy.org/
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