Belongs To

In this guide, we'll cover the BelongsTo relationship using CQL's Active Record syntax. We'll start with an Entity-Relationship Diagram (ERD) to illustrate how this relationship works and continuously build upon this diagram as we introduce new relationships in subsequent guides.

What is a BelongsTo Relationship?

The BelongsTo association in a database indicates that one entity (a record) refers to another entity by holding a foreign key to that record. For example, a Comment belongs to a Post, and each comment references the Post it is associated with by storing the post_id as a foreign key.

Example Scenario: Posts and Comments

Let's say you have a blog system where:

• A Post can have many Comments.

• A Comment belongs to one Post.

We'll start by implementing the BelongsTo relationship from the Comment to the Post.

---

Defining the Schema

We'll first define the posts and comments tables using CQL's schema DSL.

codeAcmeDB = CQL::Schema.define(
  :acme_db,
  adapter: CQL::Adapter::Postgres,
  uri: ENV["DATABASE_URL"]
) do

  table :posts do
    primary
    text :title
    text :body
    timestamp :published_at
  end

  table :comments do
    primary
    bigint :post_id
    text :body
  end
end

• posts table: Contains the blog post data (title, body, and published date).

• comments table: Contains the comment data and a foreign key post_id which references the posts table.

---

Defining the Models

Next, we'll define the Post and Comment structs in CQL.

Post Model

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

  property id : Int64?
  property title : String
  property body : String
  property published_at : Time?

  # Initializing a new post with title, body, and optional published_at
  def initialize(@title : String, @body : String, @published_at : Time? = Time.utc)
  end
end

Comment Model

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

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

  # Initializing a comment with a body. Post can be associated later.
  def initialize(@body : String, @post_id : Int64? = nil)
  end

  # Association: Each Comment belongs to one Post
  belongs_to :post, Post, :post_id
end

In the Comment model, we specify the belongs_to :post, Post, :post_id association. This links each comment to its parent post. The Comment model must have a post_id attribute (matching the foreign_key option) that stores the id of the associated Post.

---

Creating and Querying Records

Now that we have defined the Post and Comment models with a belongs_to relationship, let's see how to create and query records in CQL.

Creating Records

Option 1: Create Parent, then Child

# 1. Create and save the parent Post first
post = Post.new(title: "My First Blog Post", body: "This is the body of the post.")
post.save!

# 2. Create the Comment and associate it
#    a) By setting the foreign key directly:
comment1 = Comment.new(body: "Great post via direct FK!")
comment1.post_id = post.id # Assign the foreign key
comment1.save!
puts "Comment 1 associated with Post ID: #{comment1.post_id}, Post title: #{comment1.post.try(&.title)}"

#    b) By using the association setter:
comment2 = Comment.new(body: "Awesome article via association setter!")
comment2.post = post # Assign the Post instance to the association
comment2.save!
puts "Comment 2 associated with Post ID: #{comment2.post_id}, Post title: #{comment2.post.try(&.title)}"

Option 2: Create Child and Associated Parent Simultaneously (if needed) If you have a Comment instance and want to create its Post at the same time (less common for belongs_to primary creation flow but possible via association methods):

new_comment = Comment.new(body: "A comment for a brand new post.")
# This creates a new Post, saves it, and associates it with new_comment
created_post = new_comment.create_post(title: "Post Created Via Comment", body: "Content for post created via comment.")
new_comment.save! # Save the comment itself which now has the post_id populated

puts "New comment ID: #{new_comment.id}, associated Post ID: #{new_comment.post_id}"
puts "Title of post created via comment: #{created_post.title}"
puts "Comment's post title: #{new_comment.post.try(&.title)}"

Note: create_association (like create_post) will create and save the associated object (Post) and set the foreign key on the current object (new_comment). The current object itself (new_comment) still needs to be saved if it's new.

Option 3: Build Associated Parent (without saving parent yet)

yet_another_comment = Comment.new(body: "Comment with a built post.")
# This builds a new Post instance but does not save it to the DB yet.
# The foreign key on `yet_another_comment` is not set by `build_post`.
built_post = yet_another_comment.build_post(title: "Built, Not Saved Post", body: "Body of built post.")

# You would then save `built_post` and then `yet_another_comment` after setting association.
# built_post.save!
# yet_another_comment.post = built_post
# yet_another_comment.save!
puts "Built post title: #{built_post.title} (not yet saved)"

Querying the Associated Post from a Comment

Once we have a comment, we can retrieve the associated post using the belongs_to association.

crystalCopy code# Fetch the comment
comment = Comment.find(1)

# Fetch the associated post
post = comment.post

puts post.title  # Outputs: "My First Blog Post"

In this example, comment.post will fetch the Post associated with that Comment.

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Summary

In this guide, we've covered the basics of the belongs_to relationship in CQL. We:

• Defined the Post and Comment tables in the schema.

• Created the corresponding models, specifying the belongs_to relationship in the Comment model.

• Showed how to create and query records using the belongs_to association.

Next Steps

In the next guides, we'll build on this ERD and introduce other types of relationships like has_one, has_many, and many_to_many. Stay tuned for the next part where we'll cover the has_many relationship!

Feel free to play around with this setup and extend the models or experiment with more queries to familiarize yourself with CQL's Active Record capabilities.