Re: [Sqlalchemy-tickets] [sqlalchemy] #2734: __eq__ operator appears to not be working correctly for decorated types

[sqlalchemy <mi...@zz...>]

#2734: __eq__ operator appears to not be working correctly for decorated types
-----------------------------------+------------------------------------
      Reporter:  mcclurem          |      Owner:  zzzeek
          Type:  defect            |     Status:  closed
      Priority:  medium            |  Milestone:
     Component:  declarative       |   Severity:  no triage selected yet
    Resolution:  worksforme        |   Keywords:  TypeDecorator
Progress State:  completed/closed  |
-----------------------------------+------------------------------------
Changes (by zzzeek):

 * status:  new => closed
 * resolution:   => worksforme
 * status_field:  awaiting triage => completed/closed


Comment:

 unfortunately this is a special use case that requires additional logic,
 because SQLAlchemy coerces any "obj == None" into "obj IS NULL" - if you
 turn on SQL echoing you will see this, and it occurs before the type is
 consulted.   Because it's a lexical change to the statement, there's not
 really any way to move this consultation to a later point.

 So the `__eq__()` (and probably `__ne__()` also) here must be overridden
 specifically:

 {{{
 #!python
 from sqlalchemy import *
 from sqlalchemy.orm import *
 from sqlalchemy.ext.declarative import declarative_base

 class NullableString(TypeDecorator):
     '''Turns None into the string "Null" and back in order to prevent
 Null!=Null issues'''

     impl = String

     class comparator_factory(String.comparator_factory):
         def __eq__(self, other):
             if other is None:
                 return self.expr == "NONE"
             else:
                 return String.comparator_factory.__eq__(self, other)

     def process_bind_param(self, value, dialect):
         return "NONE" if value is None else value

     def process_result_value(self, value, dialect):
         return None if value == "NONE" else value



 Base = declarative_base()

 class A(Base):
     __tablename__ = 'a'

     id = Column(Integer, primary_key=True)
     value = Column(NullableString, nullable=False)

 e = create_engine("sqlite://", echo=True)
 Base.metadata.create_all(e)

 sess = Session(e)

 sess.add_all([A(value='x'), A(value=None)])
 sess.commit()

 assert sess.execute("select id, value from a order by id").fetchall() ==
 [(1, 'x'), (2, 'NONE')]

 assert sess.query(A).filter(A.value == 'x').one().id == 1
 assert sess.query(A).filter(A.value == None).one().id == 2
 }}}

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Ticket URL: <http://www.sqlalchemy.org/trac/ticket/2734#comment:1>
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