Conventions Were Made to be Broken
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The Rails convention for naming foreign key fields is to use the singular name of the table with a suffix of _id.
In the past few days a number of people using the Foreign Key Migrations plugin together with ActiveRecord session store have discovered that the exception to the rule is session_id in the sessions table. In this case, session_id is not a recursive relationship but an unfortunately named field.
The solution is to update the migration script and add :references => nil to the line that adds the session_id. The plugin will then ignore the column and not attempt to generate a database foreign key constraint.