Hi,
we noticed that the copying of ExcludeConstraints as part of a tometadata
invocation fails for constraints with column-based expressions. The copy
implementation of ExcludeConstraint passes the strname's of its columns to
the construction of the new constraint, which is fine for regular columns,
but not for more complex expressions.
We've added a small test case (in test_metadata) that reproduces the
failure below.
We also implemented a temporary solution that seems to fix the issue by
passing copies of the constraint's column expressions instead.
We are on version 1.3.18.
Temporary solution:
```python
#elements = [(_copy_expression(col_expr, self.table, kw['target_table']),
self.operators[col]) for col, col_expr in zip(self.columns.keys(),
self.columns.values())]
elements = [(col, self.operators[col]) for col in
self.columns.keys()]
```
Test case
```python
from sqlalchemy.dialects.postgresql import ExcludeConstraint
from sqlalchemy import column, Date, literal_column
class ToMetaDataExcludeConstraint(fixtures.TestBase, ComparesTables):
@testing.requires.check_constraints
def test_copy(self):
from sqlalchemy.testing.schema import Table
meta = MetaData()
table = Table(
"mytable",
meta,
Column("myid", Integer, Sequence("foo_id_seq"),
primary_key=True),
Column("valid_from_date", Date(), nullable=True),
Column("valid_thru_date", Date(), nullable=True),
ExcludeConstraint(
(literal_column("daterange(valid_from_date,
valid_thru_date, '[]')"), '&&'),
where=column("valid_from_date") <=
column("valid_thru_date"),
name='ex_mytable_valid_date_range',
deferrable=True, initially='deferred'
)
)
meta.create_all(testing.db)
try:
meta2 = MetaData()
table_c = table.tometadata(meta2)
finally:
meta.drop_all(testing.db)
```
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