python - SQLAlchemy: transitive unique constraint -

i'm working on simple document management system each document can have number of versions , optionally number of tags point towards specific versions. users employ tags indicate version of document ready qa, production use, etc. each document, there should @ 1 tag of given name. i'm having trouble describing transitive relationship documents have versions , tags point versions, tag names never occur more once each document. here declarative tables far:

class document(base):     __tablename__ = 'documents'      id = column(integer, primary_key=true)     name = column(string, nullable=false)      versions = relationship('version', back_populates='document')     tags = relationship('tag', secondary='versions')   class version(base):     __tablename__ = 'versions'      id = column(integer, primary_key=true)     document_id = column(integer, foreignkey('document.id'))     name = column(string, nullable=false)     contents = column(string, nullable=false)      document = relationship('document', back_populates='versions')     tags = relationship('tag', secondary='version')    class tag(base):      __tagname__ = 'tags'       id = column(integer, primary_key=true)      version_id = column(integer, foreignkey('versions.id'))      name = column(string, nullable=false)       version = relationship('version', back_populates='tags')      document = relationship('document', secondary='versions') 

i tried using uniqueconstraint on tag's document , name, didn't allow me use document relationship column in constraint. read through the constraint tutorial, didn't find wanted.