Set Up Belongs To

This guide shows you how to set up a belongs_to relationship where one model references another via a foreign key.

When to Use

Use belongs_to when:

• A Comment belongs to a Post

• An Order belongs to a User

• A Photo belongs to an Album

The model with belongs_to holds the foreign key.

Schema Setup

Create tables with a foreign key column:

schema.table :posts do
  primary :id, Int64, auto_increment: true
  column :title, String
  timestamps
end
schema.posts.create!

schema.table :comments do
  primary :id, Int64, auto_increment: true
  column :body, String
  column :post_id, Int64, null: false
  timestamps

  foreign_key [:post_id], references: :posts, references_columns: [:id]
end
schema.comments.create!

Define the Relationship

Add belongs_to to the child model:

struct Comment
  include CQL::ActiveRecord::Model(Int64)
  db_context MyDB, :comments

  property id : Int64?
  property body : String
  property post_id : Int64

  belongs_to :post, Post, :post_id

  def initialize(@body : String, @post_id : Int64)
  end
end

Access the Parent

comment = Comment.find(1)
post = comment.post  # Returns Post?

if post
  puts "Comment on: #{post.title}"
end

Create with Parent

Set Foreign Key Directly

post = Post.create!(title: "My Post")

comment = Comment.new("Great post!", post.id.not_nil!)
comment.save!

Set via Association

post = Post.create!(title: "My Post")

comment = Comment.new("Great post!", 0_i64)
comment.post = post  # Sets post_id automatically
comment.save!

Build Associated Parent

Build a parent without saving:

comment = Comment.new("My comment", 0_i64)
post = comment.build_post(title: "New Post")

# Save parent, then child
post.save!
comment.post = post
comment.save!

Create Associated Parent

Create and save parent in one step:

comment = Comment.new("My comment", 0_i64)
post = comment.create_post(title: "New Post")

comment.save!  # post_id is now set

Optional Belongs To

For optional relationships, make the foreign key nullable:

schema.table :comments do
  # ...
  column :user_id, Int64, null: true  # Optional
end

struct Comment
  # ...
  property user_id : Int64?

  belongs_to :user, User, :user_id

  def initialize(@body : String, @post_id : Int64, @user_id : Int64? = nil)
  end
end

comment = Comment.find(1)
if user = comment.user
  puts "By: #{user.name}"
else
  puts "Anonymous"
end

Verify It Works

post = Post.create!(title: "Test Post")
comment = Comment.create!(body: "Test Comment", post_id: post.id.not_nil!)

comment.post.try(&.title)  # => "Test Post"
comment.post_id == post.id  # => true

Related

• Set Up Has Many

• Set Up Has One

• Define a Model