MySQL：一个表中的两个外键引用另一个表

Question

I've come across something that seemed simple before but has me scratching my head again. I have a table for users:

user_id (PK) | username| email | something

... and a table for "views" for when one user has viewed another user:

view_id (PK) | viewer_id | viewed_id | view_date

The "viewer_id" and "viewed_id" are both user_ids, allowing me to search separately for instances when a user was the viewer or the one being viewed.

I initially thought that both of these columns would be foreign keys, but having created the tables in my schema.yml file (I'm using Doctrine 1.2) and specified two separate foreign relationships (one for each column), it seems Doctrine only takes into account the first listed foreign relationship between these two tables (user_id > viewer_id).

It's got me confused now whether this is correct MySQL behaviour, a problem in Doctrine, or a problem in the way I'm approaching this, or nothing to worry about! Can there be two separate foreign keys from one table mapped to the same column in another table? Is it even logical, given that a JOIN would still give me access to "views" through a user_id? Have I got it wrong?

Thanks for your time.

EDIT - The schema file:

User:
relations:
View: {class: View, local: user_id, foreign: viewer_id, type: many, foreignType: one, alias: View, foreignAlias: User}
View: {class: View, local: user_id, foreign: viewed_id, type: many, foreignType: one, alias: View, foreignAlias: User}

... only difference is viewer_id/viewed_id

Answer 1

And here we go: You specified the same aliases for the relations.

User:
  relations:
    viewed_by: 
       class: View
       local: user_id
       foreign: viewed_id
       type: many
       foreignType: one
       foreignAlias: viewed

    viewed:
      class: View
      local: user_id
      foreign: viewer_id
      type: many
      foreignType: one
      foreignAlias: viewer

Or you set up the whole many-to-many relation differently:

User:
   relations:
     viewed_by: 
       class: User 
       local: viewed_id
       foreign: viewer_id,
       refClass: View
     viewed:
       class: User
       local:viewer_id
       foreign: viewed_id
       refClass: View

and View should look like

View:
  columns:
    viewed_id:
      type: integer
      primary: true
    viewer_id:
      type: integer
      primary: true

See the Doctrine documentation on many-to-many relationships .

Answer 2

Mysql allows multiple foreign keys on the same table, even to the same column in another table, but I cannot tell why doctrine only creates one of them:

mysql> CREATE TABLE test1 (id INT);
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE test3 (ref1 INT, ref2 INT, FOREIGN KEY (ref1) REFERENCES test1(id), FOREIGN KEY (ref2) REFERENCES test1(id));
Query OK, 0 rows affected (0.01 sec)