Has Many

In this guide, we’ll focus on the HasMany relationship using CQL's Active Record syntax. Like the previous BelongsTo and HasOne relationships, we’ll start with an Entity-Relationship Diagram (ERD) to visually explain how the HasMany relationship works and build on our previous schema.

What is a HasMany Relationship?

The HasMany relationship indicates that one entity (a record) is related to multiple other entities. For example, a Post can have many Comments. This relationship is a one-to-many mapping between two entities.

Example Scenario: Posts and Comments

In a blogging system:

• A Post can have many Comments.

• Each Comment belongs to one Post.

This is a common one-to-many relationship where one post can have multiple comments, but each comment refers to only one post.

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Defining the Schema

We’ll define the posts and comments tables in the schema using CQL’s DSL.

AcmeDB = Cql::Schema.db_context(
  :acme_db,
  adapter: Cql::Adapter::Postgres,
  uri: ENV["DATABASE_URL"]
) do
  table :posts do
    primary :id, Int64, auto_increment: true
    text :title
    text :body
    timestamp :published_at
  end

  table :comments do
    primary :id, Int64, auto_increment: true
    bigint :post_id
    text :body
  end
end

• posts table: Stores post details like title, body, and published_at.

• comments table: Stores comment details with a foreign key post_id that references the posts table.

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Defining the Models

Let’s db_context the Post and Comment models and establish the HasMany and BelongsTo relationships in CQL.

Post Model

struct Post < Cql::Record(Post, Int64)
  db_context AcmeDB, :posts

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

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

  # Association: A Post has many Comments
  has_many :comments, Comment
end

• The has_many :comments association in the Post model defines that each post can have multiple comments.

Comment Model

struct Comment< Cql::Record(Comment, Int64)
  db_context AcmeDB, :comments

  getter id : Int64?
  getter post_id : Int64
  getter body : String

  # Initializing a comment with post_id (foreign key) and body
  def initialize(@post_id : Int64, @body : String)
  end

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

• The belongs_to :post association in the Comment model links each comment to a post by using the post_id foreign key.

Creating and Querying Records

Now that we have defined the Post and Comment models with a HasMany and BelongsTo relationship, let’s create and query records in CQL.

Creating a Post and Comments

# Create a new Post
post = Post.new("My First Blog Post", "This is the content of my first blog post.")
post.save

# Create Comments for the Post
comment1 = Comment.new(post.id.not_nil!, "Great post!")
comment2 = Comment.new(post.id.not_nil!, "Thanks for sharing.")
comment1.save
comment2.save

• First, we create a Post and save it to the database.

• Then, we create two Comments and associate them with the post by passing post.id as the post_id for each comment.

Accessing Comments from the Post

Once a post has comments, you can retrieve all the comments using the HasMany association.

# Fetch the post
post = Post.find(1)

# Fetch all associated comments
post.comments.each do |comment|
  puts comment.body
end

Here, post.comments retrieves all the comments associated with the post, and we loop through them to print each comment’s body.

Accessing the Post from a Comment

You can also retrieve the post associated with a comment using the BelongsTo association.

# 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 fetches the post that the comment belongs to.

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Updating and Deleting the Associations

Adding a New Comment to an Existing Post

You can add a new comment to an existing post as follows:

# Fetch the post
post = Post.find(1)

# Create a new comment for the post
new_comment = Comment.new(post.id.not_nil!, "Another comment")
new_comment.save

Deleting a Post and Its Associated Comments

If you delete a post, you may want to delete all associated comments as well. However, by default, this will not happen unless you specify cascade deletion in your database.

# Fetch the post
post = Post.find(1)

# Delete the post
post.delete

# (Optional) Manually delete the associated comments
post.comments.each do |comment|
  comment.delete
end

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Advanced Querying

You can also perform advanced queries using the HasMany relationship. For example, finding posts with a certain number of comments or filtering comments for a post based on specific conditions.

Fetching Posts with Comments

You can load posts along with their comments in one query:

posts_with_comments = Post.includes(:comments).all

Finding Comments for a Specific Post

If you want to query for specific comments associated with a post, you can filter them as follows:

# Fetch the post
post = Post.find(1)

# Find comments with specific condition (e.g., containing the word "Great")
filtered_comments = post.comments.where { body.like("%Great%") }

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Summary

In this guide, we’ve explored the HasMany relationship in CQL. We:

• Defined the Post and Comment tables in the schema.

• Created corresponding models, specifying the HasMany relationship in the Post model and the BelongsTo relationship in the Comment model.

• Demonstrated how to create, query, update, and delete records using the HasMany and BelongsTo associations.

Next Steps

In the next guide, we’ll build upon this ERD and cover the ManyToMany relationship, which is useful when two entities are associated with many of each other (e.g., a post can have many tags, and a tag can belong to many posts).

Feel free to experiment with the HasMany relationship by adding more fields, filtering queries, or extending your schema to handle more complex use cases.