Set Up Has One

This guide shows you how to set up a has_one relationship where one model has exactly one related model.

When to Use

Use has_one when:

• A User has one Profile

• An Order has one ShippingAddress

• A Company has one Address

The other model holds the foreign key.

Schema Setup

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

schema.table :profiles do
  primary :id, Int64, auto_increment: true
  column :user_id, Int64, null: false
  column :bio, String
  column :avatar_url, String

  foreign_key [:user_id], references: :users, references_columns: [:id]
  index [:user_id], unique: true
  timestamps
end
schema.profiles.create!

Note: The unique index ensures only one profile per user.

Define the Relationship

Add has_one to the parent model:

struct User
  include CQL::ActiveRecord::Model(Int64)
  db_context MyDB, :users

  property id : Int64?
  property name : String

  has_one :profile, Profile, :user_id

  def initialize(@name : String)
  end
end

struct Profile
  include CQL::ActiveRecord::Model(Int64)
  db_context MyDB, :profiles

  property id : Int64?
  property user_id : Int64
  property bio : String?
  property avatar_url : String?

  belongs_to :user, User, :user_id

  def initialize(@user_id : Int64, @bio : String? = nil, @avatar_url : String? = nil)
  end
end

Access the Related Record

user = User.find(1)
profile = user.profile  # Returns Profile?

if profile
  puts "Bio: #{profile.bio}"
else
  puts "No profile"
end

Create Related Record

Create Profile for User

user = User.create!(name: "John")

profile = Profile.create!(
  user_id: user.id.not_nil!,
  bio: "Hello world",
  avatar_url: "/avatars/john.jpg"
)

user.profile.try(&.bio)  # => "Hello world"

Build and Save

user = User.find(1)

profile = Profile.new(user.id.not_nil!, "My bio")
profile.save!

Update Related Record

user = User.find(1)

if profile = user.profile
  profile.bio = "Updated bio"
  profile.save!
end

Delete Related Record

user = User.find(1)
user.profile.try(&.delete!)

Verify It Works

user = User.create!(name: "Test User")

profile = Profile.create!(
  user_id: user.id.not_nil!,
  bio: "Test bio"
)

user.profile.try(&.bio)  # => "Test bio"
profile.user.try(&.name)  # => "Test User"

Related

• Set Up Belongs To

• Set Up Has Many

• Define a Model