Many To Many

ManyToMany Relationship Guide

In this guide, we’ll cover the ManyToMany relationship using CQL's Active Record syntax. This is a more complex relationship compared to HasOne and HasMany, and it’s commonly used when two entities have a many-to-many relationship, such as posts and tags where:

• A Post can have many Tags.

• A Tag can belong to many Posts.

To model this, we need an intermediate (or join) table that connects these two entities.

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What is a ManyToMany Relationship?

A ManyToMany relationship means that multiple records in one table can relate to multiple records in another table. For example:

• A Post can have multiple Tags (e.g., "Tech", "News").

• A Tag can belong to multiple Posts (e.g., both Post 1 and Post 2 can have the "Tech" tag).

Example Scenario: Posts and Tags

We’ll use a scenario where:

• A Post can have many Tags.

• A Tag can belong to many Posts.

We will represent this many-to-many relationship using a join table called PostTags.

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Defining the Schema

We’ll define the posts, tags, and post_tags tables in the schema using CQL’s DSL.

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

  table :tags do
    primary
    text :name
  end

  table :post_tags do
    primary
    bigint :post_id, index: true
    bigint :tag_id, index: true
    index [:post_id, :tag_id], unique: true
  end
end

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

• tags table: Stores tag names.

• post_tags table: A join table that connects posts and tags via their foreign keys post_id and tag_id.

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Defining the Models

Let’s define the Post, Tag, and PostTag models in CQL, establishing the ManyToMany relationship.

Post Model

struct Post < Cql::Record(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 Tags through PostTags
  has_many :post_tags, PostTag
  has_many :tags, Tag, through: :post_tags
end

In the Post model, we define:

• has_many :post_tags to establish the association between Post and the join table PostTag.

• has_many :tags, through: :post_tags to associate Post with Tag through the join table.

Tag Model

struct Tag < Cql::Record(Int64)
  db_context AcmeDB, :tags

  getter id : Int64?
  getter name : String

  # Initializing a new tag with name
  def initialize(@name : String)
  end

  # Association: A Tag has many Posts through PostTags
  has_many :post_tags, PostTag
  has_many :posts, Post, through: :post_tags
end

Similarly, in the Tag model, we db_context:

• has_many :post_tags to associate Tag with PostTag.

• has_many :posts, through: :post_tags to associate Tag with Post through the join table.

PostTag Model (Join Table)

struct PostTag < Cql::Record(Int64)
  db_context AcmeDB, :post_tags

  getter id : Int64?
  getter post_id : Int64
  getter tag_id : Int64

  # Initializing a PostTag with post_id and tag_id
  def initialize(@post_id : Int64, @tag_id : Int64)
  end

  # Associations
  belongs_to :post, Post
  belongs_to :tag, Tag
end

The PostTag model links each Post and Tag by storing their respective IDs.

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Creating and Querying Records

Now that we’ve db_contextd the Post, Tag, and PostTag models, let’s explore how to create and query records in a ManyToMany relationship.

Creating a Post with Tags

# Create a new Post
post = Post.new("Crystal Language Guide", "This is a guide about Crystal.")
post.save

# Create some Tags
tag1 = Tag.new("Tech")
tag2 = Tag.new("Programming")
tag1.save
tag2.save

# Associate the Post with the Tags
PostTag.new(post.id.not_nil!, tag1.id.not_nil!).save
PostTag.new(post.id.not_nil!, tag2.id.not_nil!).save

In this example:

• We create a Post and save it to the database.

• We create two Tags ("Tech" and "Programming") and save them.

• We create records in the PostTag join table to associate the Post with these two Tags.

Accessing Tags for a Post

Once a post has tags associated with it, you can retrieve them using the ManyToMany association.

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

# Fetch all associated tags
post.tags.each do |tag|
  puts tag.name
end

Here, post.tags retrieves all the tags associated with the post.

Accessing Posts for a Tag

Similarly, you can retrieve all posts associated with a tag.

# Fetch the tag
tag = Tag.find(1)

# Fetch all associated posts
tag.posts.each do |post|
  puts post.title
end

Here, tag.posts retrieves all the posts associated with the tag.

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Updating and Deleting the Associations

Adding New Tags to a Post

You can associate more tags with an existing post by creating new entries in the PostTag join table.

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

# Create a new tag and associate it with the post
new_tag = Tag.new("Web Development")
new_tag.save

# Associate the post with the new tag
PostTag.new(post.id.not_nil!, new_tag.id.not_nil!).save

Removing a Tag from a Post

To disassociate a tag from a post, you need to delete the corresponding record from the PostTag join table.

# Fetch the post and tag
post = Post.find(1)
tag = Tag.find(2)

# Find the PostTag entry
post_tag = PostTag.where(post_id: post.id, tag_id: tag.id).first

# Delete the PostTag entry
post_tag.delete

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Advanced Querying

You can also perform advanced queries using the ManyToMany relationship, such as finding posts with a specific tag or fetching tags for multiple posts.

Fetching Posts with a Specific Tag

To find all posts associated with a specific tag, you can filter the posts by the tag name.

# Fetch the tag
tag = Tag.where { name.eq("Tech") }.first

# Fetch all posts associated with the "Tech" tag
tag.posts.each do |post|
  puts post.title
end

Fetching Tags for Multiple Posts

You can fetch all tags associated with multiple posts as follows:

# Fetch posts
posts = Post.where { id.in([1, 2, 3]) }

# Fetch all tags associated with the posts
posts.each do |post|
  post.tags.each do |tag|
    puts "Post #{post.title} has tag #{tag.name}"
  end
end

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Summary

In this guide, we explored the ManyToMany relationship in CQL. We:

• Define the Post, Tag, and PostTag tables in the schema

• Created corresponding models, specifying the ManyToMany relationship between Post and Tag through the PostTag join table.

• Demonstrated how to create, query, update, and delete records in a ManyToMany relationship.

Next Steps

This concludes our series of guides on relationships in CQL Active Record, covering BelongsTo, HasOne, HasMany, and ManyToMany. Feel free to experiment by extending your models, adding validations, or implementing more complex queries to suit your needs.