marshmallow-sqlalchemy-referential

import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base
from marshmallow_sqlalchemy_referential import Referential
from marshmallow_sqlalchemy import ModelSchema
from marshmallow import fields

Base = declarative_base()

user_group_assoc = sa.Table(
  'user_group_assoc', Base.metadata,
  sa.Column('user_id', sa.Integer, sa.ForeignKey('user.id')),
  sa.Column('group_id', sa.Integer, sa.ForeignKey('group.id'))
)

class User(Base):
    __tablename__ = 'user'

    id = sa.Column(sa.Integer(), primary_key=True)
    groups = sa.orm.relationship('Group', secondary=user_group_assoc)

class Group(Base):
    __tablename__ = 'group'

    id = sa.Column(sa.Integer(), primary_key=True)
    users = sa.orm.relationship('User', secondary=user_group_assoc)


class GroupSchema(ModelSchema):
  class Meta():
      dump_only = ['id']

  id = fields.Integer()

class UserSchema(ModelSchema):
  class Meta():
      dump_only = ['id']

  id = fields.Integer()
  groups = Referential(GroupSchema, model=Group, many=True)