The association struct for has_one and has_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
• related_key - The key on the related 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 is :child
• preload_order - Default order_by of the association, used only by preload