has_and_belongs_to_many连接表中不允许使用主键

Question

I have many-to-many linking table between two entities, datacenters and projects, in legacy code. I discovered, that it's really a one to many relationship.

As a first step to cleaning up the relationship, I wanted to put a unique index on one of the fields.

Now I'm getting the following error:

Primary key is not allowed in a has_and_belongs_to_many join table (datacenters_projects).

I'm using Rails 2.3.8 running on ruby 1.8.7p174 and mysql Ver 14.12 Distrib 5.0.77.

This is not a primary key, it's just a unique index. I suppose it's a candidate key, but I haven't decided it is a primary key.

I don't know why mysql considers this to be a primary key. I don't know why ActiveRecord would object to the linking table in a has_and_belongs_to_many having a primary key.

But my real question is, "Is there any way I can put a unique index on one of the fields, without ActiveRecord giving me an error?"

Is there any way I can tell add_index that this is not a primary key?

Answer 1

I'm not sure why that would fail. You should be able to get around it using SQL in your migration though, eg:

ActiveRecord::Base.connection.execute("CREATE UNIQUE INDEX project_id_index ON datacenters_projects (project_id)")

Answer 2

Switch to

has_many :through => ...

Ref: guides