Relationships

• Has Many
• Belongs To
• Has Many Through

In Coast relationships allow you to do more sophisticated queries than you would be able to otherwise.

If you never find yourself needing pull queries you can skip this section altogether.

Basic Example

Here's an example with two database tables: customer and purchase.

To add the relationship, head over to db/associations.clj:

(ns associations
  (:require [coast.db.associations :refer [table belongs-to has-many tables]]))

(defn associations []
  (tables
    (table :customer
      (has-many :purchases))

    (table :purchase
      (belongs-to :customer))))

This defines a relationship between :customer and :purchase where a customer has many purchases and the purchase table has a customer column that references the customer.id column.

NOTE: has-many attempts to make the keyword you specify singular to match the underlying singular database table name.

If you have a different table name or a table name that can't be turned into a singular, you can specify the table name:

(table :customer
  (has-many :stuff-bought :table-name "purchase"))

You can also specify the foreign key column as well:

(table :customer)
  (has-many :purchases :foreign-key "id"))

Belongs To

Belongs to represents tables with columns referencing another table.

Given these two tables:

• member
  • id
  • screen_name
  • updated_at
  • created_at
• micropost
  • id
  • member references member(id)
  • updated_at
  • created_at

    (table :micropost
      (belongs-to :member))
    

You can specify a table name that won't be singularized as well:

(table :micropost
  (belongs-to :person :table-name "member"))

You can also specify the foreign key as well

(table :micropost
  (belongs-to :member :foreign-key "id"))

Has Many Through

You can create a "many to many" relationship with has-many :through.

Here's an example, with four tables

• author
  • id
  • screen_name
  • updated_at
  • created_at
• post
  • id
  • author
  • title
  • body
  • updated_at
  • created_at
• tag
  • id
  • name unique
  • updated_at
  • created_at
• tagged
  • id
  • post
  • tag
  • updated_at
  • created_at

    ; db/associations.clj
    (ns associations
      (:require [coast.db.associations :refer [table belongs-to has-many tables]]))
    
    (defn associations []
      (tables
        (table :author
          (has-many :posts))
    
        (table :post
          (belongs-to :author)
          (has-many :tagged)
          (has-many :tags :through :tagged))
    
        (table :tag
          (has-many :tagged)
          (has-many :posts :through :tagged))
    
        (table :tagged
          (belongs-to :post)
          (belongs-to :tag))))
    

Has many through allows you to shortcut through database tables and pull data out in this way:

(coast/pull '[author/screen-name
              {:author/posts [post/title post/body
                              {:post/tags [tag/name]}]}]
            {:author/id 1})

For more information on pull syntax and how it works and relates to db/associations.clj check out the pull syntax doc