如何在SQLAlchemy列/外键mixin中指定PostgreSQL架构？

package
    __init__.py
    tables_in_schema1.py
    tables_in_schema2.py

元数据和引擎对象作为db.Model从包的顶部进入每个文件.例如,tables_in_schema1.py将具有(忽略必要的ORM导入,然后需要后向引用)表alpha：

from package import db

class TableAlpha(db.Model,object):
    __tablename__ = "alpha"
    __table_args__ = ({"schema": "schema1"})

    id_alpha = Column(INTEGER,Sequence("pk_alpha",1,1),primary_key=True)


class MixinAlphaRelation(object):

    @declared_attr
    def id_alpha(cls):
        return Column(INTEGER,ForeignKey("schema1.alpha.id_alpha"))

现在在tables_in_schema2.py中,定义了两个表.一个是独立的表,称为beta,另一个是alpha和beta之间的一对多关系的链接表,称为表rho：

from package import db
from package.tables_in_schema1 import MixinAlphaRelation

class TableBeta(db.Model,object):
    __tablename__ = "beta"
    __table_args__ = ({"schema": "schema2"})

    id_beta = Column(INTEGER,Sequence("pk_beta",primary_key=True)


class MixinBetaRelation(object):

    @declared_attr
    def id_beta(cls):
        return Column(INTEGER,ForeignKey("schema2.beta.id_beta"))


class TableRho(db.Model,MixinAlphaRelation,MixinBetaRelation):
    __tablename__ = "rho"
    __table_args__ = (
        UniqueConstraint("id_alpha","id_beta",name="uq_rho_alpha-beta"),{"schema": "schema2"})

    id_row = Column(INTEGER,Sequence("pk_rho",primary_key=True)

两个mixin的表rho继承的预期目标是生成一个由三行组成的表(并将mixin重用于其他表,这些表也引用了alpha或beta)：

CREATE TABLE schema2.rho (
    id_row INTEGER PRIMARY KEY,id_alpha INTEGER REFERENCES schema1.alpha(id_alpha),id_beta INTEGER REFERENCES schema2.beta(id_beta)
    );

CREATE INDEX uq_rho_alpha-beta ON schema2.rho(id_alpha,id_beta);

但是,当尝试通过调用db.create_all()重新创建所有这些表时,SQLAlchemy将发出错误：

sqlalchemy.exc.NoReferencedTableError: Foreign key associated with
column ‘rho.id_alpha’ could not find table ‘schema2.alpha’ with
which to generate a foreign key to target column ‘id_alpha’

似乎不是在导入的mixin中指定的表格中找到表格alpha,而是SQLAlchemy似乎在schema2中寻找它.

怎么解决这个问题？有没有办法传递/强制mixin的正确架构？谢谢.