UnboundExecutionError for joint table subclasses when binds is constructed from list of tables

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:

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?

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