Connect to multiple data sources in same app

首先跟踪 GraphQLView 对 get_context 的处理方式，以及 SQLAlchemyConnectionField 如何获取其 session，然后将所请求的行为与 Graphene-SQLAlchemy 进行比较。该 issue 未提供可作为目标的仓库文件或测试。完成的标准是：多个已配置的数据源可以在同一个 app 中工作，同时保留 SQLAlchemyConnectionField 及相关扩展，而无需针对 resolver 的 session 变通方案。

It appears I can only bind a single SQLAlchemy session to an app at once via the context variable:

from flask import Flask
from flask_graphql import GraphQLView
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, scoped_session
from schema import schema

# connect to database sources
engine1 = create_engine('sqlite:///db1.sqlite3')
engine2 = create_engine('sqlite:///db2.sqlite3')
session1 = scoped_session(sessionmaker(bind=engine1))
session2 = scoped_session(sessionmaker(bind=engine2))

# create app and add GraphiQL route
app = Flask(__name__)
app.add_url_rule('/graphql', view_func=GraphQLView.as_view(
    'graphql',
    schema=schema,
    graphiql=True,
    get_context=lambda: {'session': session1}
))

Is there any way to connect to more than one data source at once or is this not possible?

If not, Flask-SQLAlchemy handles this well by binding the data source name to the sqlalchemy model. I'd recommend doing something similar: http://flask-sqlalchemy.pocoo.org/2.3/binds/

I figured out a workaround by specifying the right session in the GraphQL query but this seems very hacky and prevents me from using nice extensions like Graphene-SQLAlchemy

class Query(ObjectType):
    node = relay.Node.Field()

    # requires session1 (both resolvers work because default session is session1)
    all_employees = SQLAlchemyConnectionField(EmployeeConnections)
    employees = graphene.List(EmployeeNode, name=graphene.String())

    # requires session2
    all_departments = SQLAlchemyConnectionField(DepartmentConnections)  # can't do this because default session is session1
    departments = graphene.List(DepartmentNode, name=graphene.String())

    def resolve_departments(self, info, **kwargs):
        name = kwargs.get('name')
        if not name:
            raise GraphQLError('Name argument is required.')
        return session2.query(Department).filter_by({'name': kwargs.get('name')}).all()