The association struct for a belongs_to association.

Its fields are:

• cardinality - The association cardinality
• field - The name of the association field on the model
• owner - The model where the association was defined
• related - The model that is associated
• owner_key - The key on the owner model used for the association
• related_key - The key on the related model used for the association
• queryable - The real query to use for querying association
• defaults - Default fields used when building the association