Set Up Has Many

This guide shows you how to set up a has_many relationship where one model has multiple related records.

When to Use

Use has_many when:

• A User has many Posts

• A Post has many Comments

• A Category has many Products

Schema Setup

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

schema.table :posts do
  primary :id, Int64, auto_increment: true
  column :user_id, Int64, null: false
  column :title, String
  column :body, String
  timestamps

  foreign_key [:user_id], references: :users, references_columns: [:id]
  index [:user_id]
end
schema.posts.create!

Define the Relationship

Add has_many to the parent model:

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

  property id : Int64?
  property name : String

  has_many :posts, Post, :user_id

  def initialize(@name : String)
  end
end

struct Post
  include CQL::ActiveRecord::Model(Int64)
  db_context MyDB, :posts

  property id : Int64?
  property user_id : Int64
  property title : String
  property body : String

  belongs_to :user, User, :user_id

  def initialize(@title : String, @body : String, @user_id : Int64)
  end
end

Query Related Records

Get All Related Records

user = User.find(1)
posts = user.posts.all

posts.each do |post|
  puts post.title
end

Count Related Records

user = User.find(1)
count = user.posts.count
puts "User has #{count} posts"

Filter Related Records

user = User.find(1)

# Published posts only
published = user.posts.where(published: true).all

# Most recent posts
recent = user.posts.order(created_at: :desc).limit(5).all

Create Related Records

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

# Create posts for user
post1 = Post.create!(
  title: "First Post",
  body: "Hello world",
  user_id: user.id.not_nil!
)

post2 = Post.create!(
  title: "Second Post",
  body: "Another post",
  user_id: user.id.not_nil!
)

user.posts.count  # => 2

Delete Related Records

Delete Specific Posts

user = User.find(1)
user.posts.where(published: false).each(&.delete!)

Delete All Posts

user = User.find(1)
user.posts.all.each(&.delete!)

Cascade Delete with Foreign Keys

Configure foreign key to cascade deletes:

schema.table :posts do
  # ...
  foreign_key [:user_id], references: :users, references_columns: [:id], on_delete: :cascade
end

Now when a user is deleted, their posts are automatically deleted.

Check for Related Records

user = User.find(1)

if user.posts.count > 0
  puts "User has posts"
else
  puts "User has no posts"
end

Verify It Works

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

Post.create!(title: "Post 1", body: "Body 1", user_id: user.id.not_nil!)
Post.create!(title: "Post 2", body: "Body 2", user_id: user.id.not_nil!)

user.posts.count  # => 2
user.posts.all.map(&.title)  # => ["Post 1", "Post 2"]

Related

• Set Up Belongs To

• Set Up Has One

• Set Up Many-to-Many