定义没有外键的关系

Question

How can I make a relationship without having a foreign key?

@declared_attr 
def custom_stuff(cls): 
    joinstr = 'foreign(Custom.name) == "{name}"'.format(name=cls.__name__)  
    return db.relationship('Custom', primaryjoin=joinstr) 

This raises an error:
ArgumentError: Could not locate any simple equality expressions involving locally mapped foreign key columns for primary join condition

This works, but I think it's a pretty ugly hack.

@declared_attr
def custom_stuff(cls):
    joinstr = 'or_(
                  and_(foreign(Custom.name) == MyTable.title, 
                       foreign(Custom.name) != MyTable.title), 
                  foreign(Custom.name) == "{name}")'.format(name=cls.__name__)
    return db.relationship('Custom', primaryjoin=joinstr)

Is there a better way to do this?

EDIT: the extra attribute needs to be added as @declared_attr and has to use a relationship, since our serializer is written so it works with declarred attrs.

Doing this with @hybrid_property or something else would work, but then our json serializer would break. Getting that to work seems harder than defining a relationship.

Answer 1

You don't necessarily have to define relationships when creating tables in a database (this applies for almost every SQL). You can still join tables that don't have a foreign-key relation predefined (the main reason for foreign keys is to enforce data consistency, not to define what you can join or not).

See this for reference - http://docs.sqlalchemy.org/en/latest/orm/query.html#sqlalchemy.orm.query.Query.join

If you would still like to "show" the database and table structure/model, then better use some entity relationship modeler like ERwin (or some diagramming software).

Answer 2

Maybe you should point to the meaning of "relationship" word. It states that some thing "depends" on some other thing or both depend each other. If you're trying to define a relationship on the ERM (Entity Relationship Model) that means you should state which one of the entities "depends" on the other one. Also, databases have some hacks to deal faster with tables relating each other than just simple tables that are related on an upper abstract way. Is there any reason why you need to do that?