The association struct for many_to_many associations.

Its fields are:

• cardinality - The association cardinality
• field - The name of the association field on the schema
• owner - The schema where the association was defined
• related - The schema that is associated
• owner_key - The key on the owner schema used for the association
• queryable - The real query to use for querying association
• on_delete - The action taken on associations when schema is deleted
• on_replace - The action taken on associations when schema is replaced
• defaults - Default fields used when building the association
• relationship - The relationship to the specified schema, default :child
• join_keys - The keyword list with many to many join keys
• join_through - Atom (representing a schema) or a string (representing a table) for many to many associations
• join_defaults - A list of defaults for join associations