Trying to get filtering and ordering to work with hybrid properties in single table inheritance
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Tucker Beck
Oct 30, 2017, 6:43:11 PM10/30/17
to sqlalchemy
I wrestled through getting a model heirarchy to work with single-table inheritance that is polymorphic on a hybrid attribute on this mailing list a while ago.
The problem I'm running into now is that it doesn't seem to work correctly when I want to use the hybrid property for filtering or ordering.
This seems to be an issue with auto-correlation, but I can't seem to figure out how to get it working.
Here is a runnable example:
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base, declared_attr
from sqlalchemy.ext.hybrid import hybrid_property, Comparator
Base = declarative_base()
class classproperty(property):
"""A decorator that behaves like @property except that operates
on classes rather than instances.
The decorator is currently special when using the declarative
module, but note that the
:class:`~.sqlalchemy.ext.declarative.declared_attr`
decorator should be used for this purpose with declarative.
"""
def __init__(self, fget, *arg, **kw):
super(classproperty, self).__init__(fget, *arg, **kw)
self.__doc__ = fget.__doc__
def __get__(desc, self, cls):
return desc.fget(cls)
class ModelBase(Base):
__abstract__ = True
def __repr__(self):
class HybridType(ModelBase):
__tablename__ = 'hybrid_types'
id = Column(Integer, primary_key=True)
name = Column(Text)
class HybridModel(ModelBase):
__tablename__ = 'hybrids'
id = Column(Integer, primary_key=True)
name = Column(Text)
hybrid_type_id = Column(Integer, ForeignKey('hybrid_types.id'), nullable=False)
hybrid_type = relationship('HybridType')
def __init__(self, *args, **kwargs):
self.hybrid_type_name = self.hybrid_type_identity
return super().__init__(*args, **kwargs)
@classproperty
def hybrid_type_identity(cls):
return cls.__name__
@declared_attr
def __mapper_args__(cls):
return dict(
polymorphic_on=cls.hybrid_type_name_subquery(),
polymorphic_identity=cls.hybrid_type_identity,
)
@hybrid_property
def hybrid_type_name(self):
return self.hybrid_type.name
@hybrid_type_name.setter
def hybrid_type_name(self, value):
self.hybrid_type_id = (
select([HybridType.id]).
where(HybridType.name == value)
)
@hybrid_type_name.expression
def hybrid_type_name(cls):
return cls.hybrid_type_name_subquery()
@classmethod
def hybrid_type_name_subquery(cls):
return select([HybridType.name]).where(HybridType.id == cls.hybrid_type_id).as_scalar()
class HybridComparator(Comparator):
def operate(self, op, other):
return op(HybridType.id, select([HybridType.id]).where(HybridType.name == other).as_scalar())
@hybrid_type_name.comparator
def hybrid_type_name(cls):
return cls.HybridComparator(cls)
class HybridAlpha(HybridModel):
pass
class HybridBeta(HybridModel):
pass
e = create_engine("sqlite://", echo=False)
Base.metadata.create_all(e)
session = Session(e)
session.add(HybridType(name=HybridAlpha.hybrid_type_identity))
session.add(HybridType(name=HybridBeta.hybrid_type_identity))
session.add(HybridAlpha(name='alpha_instance'))
session.add(HybridBeta(name='beta_instance'))
print("--- Test query from base hybrid model ---")
assert session.query(HybridModel).count() == 2
print("passed")
print("--- Test query from base derived hybrid model ---")
assert session.query(HybridAlpha).count() == 1
assert session.query(HybridBeta).count() == 1
print("passed")
print("--- Test query order_by on hybrid attribute ---")
assert [
x.hybrid_type_name for x
in session.query(HybridModel).order_by(HybridModel.hybrid_type_name)
] == [HybridAlpha.hybrid_type_identity, HybridBeta.hybrid_type_identity]
print("passed")
print("--- Test query filter_by on hybrid attribute ---")
assert session.query(HybridModel).filter_by(hybrid_type_name=HybridAlpha.hybrid_type_identity).count() == 1
print("passed")
print("--- Test query filtered on hybrid attribute ---")
assert session.query(HybridModel).filter(HybridAlpha.hybrid_type_name == HybridAlpha.hybrid_type_identity).count() == 1
print("passed")
Running this results in the following out put:
$ python demo.py
--- Test query from base hybrid model ---
passed
--- Test query from base derived hybrid model ---
passed
--- Test query order_by on hybrid attribute ---
Traceback (most recent call last):
File "demo.py", line 121, in <module>
in session.query(HybridModel).order_by(HybridModel.hybrid_type_name)
File "<string>", line 2, in order_by
File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/base.py", line 201, in generate
fn(self, *args[1:], **kw)
File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 1589, in order_by
criterion = self._adapt_col_list(criterion)
File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 256, in _adapt_col_list
for o in cols
File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 256, in <listcomp>
for o in cols
File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/elements.py", line 4191, in _literal_as_label_reference
return _literal_as_text(element)
File "/Users/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/elements.py", line 4230, in _literal_as_text
"instead" % type(element)
sqlalchemy.exc.ArgumentError: SQL expression object or string expected, got object of type <class 'sqlalchemy.ext.declarative.api.DeclarativeMeta'> instead
Mike Bayer
Oct 30, 2017, 7:58:11 PM10/30/17
to sqlal...@googlegroups.com
On Mon, Oct 30, 2017 at 6:43 PM, Tucker Beck <tucke...@gmail.com> wrote:
> I wrestled through getting a model heirarchy to work with single-table
> inheritance that is polymorphic on a hybrid attribute on this mailing list a
> while ago.
>
> see: https://groups.google.com/d/topic/sqlalchemy/KJXSHwbhbLA/discussion
>
> The problem I'm running into now is that it doesn't seem to work correctly
> when I want to use the hybrid property for filtering or ordering.
> This seems to be an issue with auto-correlation, but I can't seem to figure
> out how to get it working.
>
> Here is a runnable example:
OK so lets pdb:
> I wrestled through getting a model heirarchy to work with single-table
> inheritance that is polymorphic on a hybrid attribute on this mailing list a
> while ago.
>
> see: https://groups.google.com/d/topic/sqlalchemy/KJXSHwbhbLA/discussion
>
> The problem I'm running into now is that it doesn't seem to work correctly
> when I want to use the hybrid property for filtering or ordering.
> This seems to be an issue with auto-correlation, but I can't seem to figure
> out how to get it working.
>
> Here is a runnable example:
class HybridBeta(HybridModel):
pass
session = Session()
import pdb
pdb.set_trace()
what are we getting from HybridModel.hybrid_type_name:
(Pdb) HybridModel.hybrid_type_name
<sqlalchemy.orm.attributes.create_proxied_attribute.<locals>.Proxy
object at 0x7f95594ca200>
that's not right. I see there's a @comparator and also an
@expression. Those actually aren't designed to be used together,
you'd use one or the other. I'm not sure what you're trying to do
but based on the organization of what I see you'd want to do this:
@classmethod
def hybrid_type_name_subquery(cls):
return select([HybridType.name]).where(HybridType.id ==
cls.hybrid_type_id).as_scalar()
class HybridComparator(Comparator):
return self.expression.hybrid_type_name_subquery()
def operate(self, op, other):
return op(HybridType.id,
select([HybridType.id]).where(HybridType.name == other).as_scalar())
@hybrid_type_name.comparator
def hybrid_type_name(cls):
return cls.HybridComparator(cls)
print(session.query(HybridModel).order_by(HybridModel.hybrid_type_name))
SELECT hybrids.id AS hybrids_id, hybrids.name AS hybrids_name,
hybrids.hybrid_type_id AS hybrids_hybrid_type_id, (SELECT
hybrid_types.name
FROM hybrid_types
WHERE hybrid_types.id = hybrids.hybrid_type_id) AS _sa_polymorphic_on
FROM hybrids ORDER BY (SELECT hybrid_types.name
FROM hybrid_types
WHERE hybrid_types.id = hybrids.hybrid_type_id)
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
> ---
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Tucker Beck
Oct 31, 2017, 12:18:44 PM10/31/17
to sqlal...@googlegroups.com
Thanks so much for getting back so fast, Mike!
For some background: we started with the expression, but then discovered that the performance we were getting was pretty bad on queries when the table was large and we were filtering by the hybrid attribute. Postgres was doing a sequence-scan, and the plan was pretty bad. So, I learned about custom comparators, and that helped the performance for those kind of queries really dramatically. I didn't realize that the comparator was not meant to be used along-side an expression; I thought they served different purposes.
Trying your suggestion worked great for order_by. However, filtering on the hybrid attribute is failing with complaints about correlation.
I revised the code as:
@classmethod
def hybrid_type_name_subquery(cls):
return select([HybridType.name]).where(HybridType.id == cls.hybrid_type_id).as_scalar()
class HybridComparator(Comparator):
def __clause_element__(self):
return self.expression.hybrid_type_name_subquery()
def operate(self, op, other):
return op(HybridType.id, select([HybridType.id]).where(HybridType.name == other).as_scalar())
@hybrid_type_name.comparator
def hybrid_type_name(cls):
return cls.HybridComparator(cls)
And here's the output
/venv:cem/ $ python demo.py
--- Test query from base hybrid model ---
passed
--- Test query from base derived hybrid model ---
passed
--- Test query order_by on hybrid attribute ---
passed
--- Test query filter_by on hybrid attribute ---
Traceback (most recent call last):
File "demo.py", line 124, in <module>
assert session.query(HybridModel).filter_by(hybrid_type_name=HybridAlpha.hybrid_type_identity).correlate(HybridModel).count() == 1
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 3089, in count
return self.from_self(col).scalar()
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 2843, in scalar
ret = self.one()
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 2814, in one
ret = self.one_or_none()
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 2784, in one_or_none
ret = list(self)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 2855, in __iter__
return self._execute_and_instances(context)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/orm/query.py", line 2878, in _execute_and_instances
result = conn.execute(querycontext.statement, self._params)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 945, in execute
return meth(self, multiparams, params)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/elements.py", line 263, in _execute_on_connection
return connection._execute_clauseelement(self, multiparams, params)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1046, in _execute_clauseelement
if not self.schema_for_object.is_default else None)
File "<string>", line 1, in <lambda>
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/elements.py", line 436, in compile
return self._compiler(dialect, bind=bind, **kw)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/elements.py", line 442, in _compiler
return dialect.statement_compiler(dialect, self, **kw)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 435, in __init__
Compiled.__init__(self, dialect, statement, **kwargs)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 216, in __init__
self.string = self.process(self.statement, **compile_kwargs)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 242, in process
return obj._compiler_dispatch(self, **kwargs)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/visitors.py", line 81, in _compiler_dispatch
return meth(self, **kw)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 1738, in visit_select
text, select, inner_columns, froms, byfrom, kwargs)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 1817, in _compose_select_body
for f in froms])
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 1817, in <listcomp>
for f in froms])
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/visitors.py", line 81, in _compiler_dispatch
return meth(self, **kw)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 1379, in visit_alias
asfrom=True, **kwargs) + \
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/annotation.py", line 80, in _compiler_dispatch
self, visitor, **kw)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/visitors.py", line 81, in _compiler_dispatch
return meth(self, **kw)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 1716, in visit_select
for name, column in select._columns_plus_names
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 1716, in <listcomp>
for name, column in select._columns_plus_names
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 1488, in _label_select_column
**column_clause_args
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/visitors.py", line 81, in _compiler_dispatch
return meth(self, **kw)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 664, in visit_label
OPERATORS[operators.as_] + \
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/visitors.py", line 81, in _compiler_dispatch
return meth(self, **kw)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 576, in visit_grouping
return "(" + grouping.element._compiler_dispatch(self, **kwargs) + ")"
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/visitors.py", line 81, in _compiler_dispatch
return meth(self, **kw)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 1685, in visit_select
froms = self._setup_select_stack(select, entry, asfrom, lateral)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/compiler.py", line 1788, in _setup_select_stack
implicit_correlate_froms=asfrom_froms)
File "/home/tbeck/.virtualenvs/cem/lib/python3.5/site-packages/sqlalchemy/sql/selectable.py", line 2843, in _get_display_froms
"manually." % self)
sqlalchemy.exc.InvalidRequestError: Select statement 'SELECT hybrid_types.name
FROM hybrid_types, hybrids
WHERE hybrid_types.id = hybrids.hybrid_type_id' returned no FROM clauses due to auto-correlation; specify correlate(<tables>) to control correlation manually.
I've tried looking for some examples for how to use correlate and tried stuffing it in a few places and still haven't gotten around this error. What am I missing?
(Thanks again so much for the quick response)
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The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
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-=Tucker A. Beck=-
Illustrious Writer
Devious Coder
Last Hope for the Free World
Also, Modest
Illustrious Writer
Devious Coder
Last Hope for the Free World
Also, Modest
Mike Bayer
Oct 31, 2017, 1:20:04 PM10/31/17
to sqlal...@googlegroups.com
that error can be fixed using correlate_except:
@classmethod
def hybrid_type_name_subquery(cls):
return select([HybridType.name]).\
where(HybridType.id == cls.hybrid_type_id).\
correlate_except(HybridType).as_scalar()
but now that we're looking at your queries in full, the comparator
makes no sense. It just pulls in HybridType into the FROM clause and
makes a broken query with "FROM hybrids, hybrid_types" without them
being joined.
your tests pass just using plain @expression and keeping the
subqueries contained:
@hybrid_property
def hybrid_type_name(self):
return self.hybrid_type.name
@hybrid_type_name.setter
def hybrid_type_name(self, value):
self.hybrid_type_id = (
select([HybridType.id]).
where(HybridType.name == value)
)
@hybrid_type_name.expression
def hybrid_type_name(cls):
return cls.hybrid_type_name_subquery()
@classmethod
def hybrid_type_name_subquery(cls):
return select([HybridType.name]).\
where(HybridType.id == cls.hybrid_type_id).as_scalar()
for extra protection with correlation, correlate_except looks like this:
@classmethod
def hybrid_type_name_subquery(cls):
return select([HybridType.name]).\
where(HybridType.id == cls.hybrid_type_id).\
correlate_except(HybridType).as_scalar()
>> > email to sqlalchemy+...@googlegroups.com.
@classmethod
def hybrid_type_name_subquery(cls):
return select([HybridType.name]).\
where(HybridType.id == cls.hybrid_type_id).\
correlate_except(HybridType).as_scalar()
but now that we're looking at your queries in full, the comparator
makes no sense. It just pulls in HybridType into the FROM clause and
makes a broken query with "FROM hybrids, hybrid_types" without them
being joined.
your tests pass just using plain @expression and keeping the
subqueries contained:
@hybrid_property
def hybrid_type_name(self):
return self.hybrid_type.name
@hybrid_type_name.setter
def hybrid_type_name(self, value):
self.hybrid_type_id = (
select([HybridType.id]).
where(HybridType.name == value)
)
@hybrid_type_name.expression
def hybrid_type_name(cls):
return cls.hybrid_type_name_subquery()
@classmethod
def hybrid_type_name_subquery(cls):
where(HybridType.id == cls.hybrid_type_id).as_scalar()
for extra protection with correlation, correlate_except looks like this:
@classmethod
def hybrid_type_name_subquery(cls):
return select([HybridType.name]).\
where(HybridType.id == cls.hybrid_type_id).\
correlate_except(HybridType).as_scalar()
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>>
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>>
>> http://www.sqlalchemy.org/
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>>
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>> To post example code, please provide an MCVE: Minimal, Complete, and
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>
>
>
>
> --
> -=Tucker A. Beck=-
>
> Illustrious Writer
> Devious Coder
> Last Hope for the Free World
> Also, Modest
>
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>> For more options, visit https://groups.google.com/d/optout.
>
>
>
>
> --
> -=Tucker A. Beck=-
>
> Illustrious Writer
> Devious Coder
> Last Hope for the Free World
> Also, Modest
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
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>
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Tucker Beck
Oct 31, 2017, 1:33:00 PM10/31/17
to sqlal...@googlegroups.com
I'll try this out. The comparator was put in so that queries comparing the hybrid_type_name against a string would do a proper sub-select against the EntityType table. If we didn't provide the comparator, the hybrid expression was used and the resulting query was orders of magnitude slower. Does adding the correlate_except() method in there compel sqlalchemy to do a sub-select?
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>>
>> --
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>> The Python SQL Toolkit and Object Relational Mapper
>>
>> http://www.sqlalchemy.org/
>>
>> To post example code, please provide an MCVE: Minimal, Complete, and
>> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
>> description.
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Mike Bayer
Oct 31, 2017, 1:37:14 PM10/31/17
to sqlal...@googlegroups.com
the correlate_except tells it exactly what to "correlate" and what not
to, preventing that "auto-correlation" error.
try it out and then show me how you'd like the SQL to be adjusted
given a particular Query.
>> >> > email to sqlalchemy+...@googlegroups.com.
to, preventing that "auto-correlation" error.
try it out and then show me how you'd like the SQL to be adjusted
given a particular Query.
>> >> > To post to this group, send email to sqlal...@googlegroups.com.
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>> >> The Python SQL Toolkit and Object Relational Mapper
>> >>
>> >> http://www.sqlalchemy.org/
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>> >> The Python SQL Toolkit and Object Relational Mapper
>> >>
>> >> http://www.sqlalchemy.org/
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>> >
>> >
>> >
>> >
>> > --
>> > -=Tucker A. Beck=-
>> >
>> > Illustrious Writer
>> > Devious Coder
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>> > Also, Modest
>> >
>> > --
>> > SQLAlchemy -
>> > The Python SQL Toolkit and Object Relational Mapper
>> >
>> > http://www.sqlalchemy.org/
>> >
>> > To post example code, please provide an MCVE: Minimal, Complete, and
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>> >
>> >
>> > --
>> > -=Tucker A. Beck=-
>> >
>> > Illustrious Writer
>> > Devious Coder
>> > Last Hope for the Free World
>> > Also, Modest
>> >
>> > --
>> > SQLAlchemy -
>> > The Python SQL Toolkit and Object Relational Mapper
>> >
>> > http://www.sqlalchemy.org/
>> >
>> > To post example code, please provide an MCVE: Minimal, Complete, and
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>> --
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>>
>> http://www.sqlalchemy.org/
>>
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>>
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>
>
>
>
> --
> -=Tucker A. Beck=-
>
> Illustrious Writer
> Devious Coder
> Last Hope for the Free World
> Also, Modest
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
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>
>
>
>
> --
> -=Tucker A. Beck=-
>
> Illustrious Writer
> Devious Coder
> Last Hope for the Free World
> Also, Modest
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
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Tucker Beck
Nov 3, 2017, 2:44:22 PM11/3/17
to sqlalchemy
Things seem to be working with this as it is now. Adding the __clause_element__ in the comparator seems to make the need for the expression go away.
Thanks so much, Mike. I'm always impressed that you take the time to answer questions so helpfully!
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