[Sqlalchemy-tickets] Issue #3202: UnboundExecutionError for joint table subclasses when binds is constructed from list of tables (zzzeek/sqlalchemy)

[ods <iss...@bi...>]

New issue 3202: UnboundExecutionError for joint table subclasses when binds is constructed from list of tables
https://bitbucket.org/zzzeek/sqlalchemy/issue/3202/unboundexecutionerror-for-joint-table

ods:

The list of tables is the only list of mapped data that automatically collected in `MetaData` object. So it's the simplest way to construct session with multiple connections where separate connection is used for each metadata. `Session.get_bind()` does look up based on `mapper.mapped_table`. This works for simple mapped classes, but doesn't work for classes mapped to join like subclasses with joined table inheritance.

Test case:

```python
from sqlalchemy import Column, Integer, ForeignKey, create_engine, orm
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class A(Base):
    __tablename__ = 'A'
    id = Column(Integer, primary_key=True)
    type = Column(Integer, nullable=False)
    __mapper_args__ = {'polymorphic_on': type}

class B(A):
    __tablename__ = 'B'
    id = Column(ForeignKey(A.id), primary_key=True)
    __mapper_args__ = {'polymorphic_identity': 1}

engine = create_engine('sqlite://')
binds = {table: engine for table in Base.metadata.sorted_tables}
db = orm.sessionmaker(binds=binds)()

print db.get_bind(A) # Engine(sqlite://)
print db.get_bind(B) # sqlalchemy.exc.UnboundExecutionError: Could not locate a bind configured on mapper Mapper|B|B or this Session
```

Should we iterate through tables used in `Join` object `mapper.mapped_table` refers to?