Active Record

In this guide, we'll walk through using CQL with Crystal for setting up a database schema, defining records (models), establishing relationships between them, and handling migrations

Getting Started with Active Record, Relations, and Migrations in CQL

In this guide, we'll walk through using CQL with Crystal for setting up a database schema, defining records (models), establishing relationships between them, and handling migrations. This will be a foundational guide for developers who are familiar with Object-Relational Mapping (ORM) concepts from other frameworks like ActiveRecord (Rails), Ecto, or Hibernate, but are now learning CQL with Crystal.

Prerequisites

Before getting started, ensure you have the following:

Crystal language installed (latest stable version).
PostgreSQL or MySQL set up locally or in the cloud.
CQL installed in your Crystal project.

You can add CQL to your project by including it in your shard.yml:

dependencies:
  cql:
    github: azutoolkit/cql
    version: "~> 0.1.0"

Create the Record:

# src/models/user.cr
struct User < Cql::Record(Int64)
  db_context AcmeDB, :users

  property id : Int64?
  property name : String
  property email : String
  property created_at : Time
  property updated_at : Time
end

The User model is mapped to the users table. Here, we've define the fields for id, name, email, and timestamps. We also added basic validations for name and email.

Create Records: You can now create user records using the User model.
```
user = User.new(name: "John Doe", email: "john@example.com")
user.save
```
This will insert a new record into the users table.
Query Records: You can query users using the User model.
```
users = User.all
user = User.find(1)
```
User.all fetches all users, and User.find(1) fetches the user with ID 1.

Establishing Relations

CQL supports associations similar to ActiveRecord, Ecto, and other ORMs. Let's define some common relationships such as has_many and belongs_to.

Example: Users and Posts

Migration for Posts:

Create a new migration for the posts table.

crystal lib/cql/bin/cql generate migration CreatePosts

Edit the migration to add the posts table, which has a foreign key to the users table:

# db/migrate/20230817000001_create_posts.cr

class CreatePosts < Cql::Migration
  schema.table :posts do
    primary
    text :title, null: false
    text :body, null: false
    bigint :user_id, null: false, index: true
    timestamps
  end

  def up
    schema.alter :posts { AcmeDB.posts.create! }
  end

  def down
    schema.alter :posts { AcmeDB.posts.drop! }
  end
end

db_context the Post Model:

Now, let's define the Post record and establish the relationships.

# src/models/post.cr
struct Post< Cql::Record(Int64)
  db_context AcmeDB, :posts

  property id : Int64?
  property title : String
  property body : String
  property user_id : Int64
  property created_at : Time
  property updated_at : Time

  belongs_to :user, User
end

Here, the Post model includes a foreign key user_id and define a belongs_to association to the User model.

db_context the User model's association:

Update the User model to reflect the relationship with Post.

# src/models/user.cr
struct User < Cql::Record(Int64)
  db_context AcmeDB, :users

  column id : Int64?
  column name : String
  column email : String
  column created_at : Time
  column updated_at : Time

  has_many :posts, Post
end

This define a has_many association on User so that each user can have multiple posts.

Working with Relations:

Create a user and associate posts with them:

user = User.create(name: "Jane Doe", email: "jane@example.com")
post = Post.new(title: "First Post", body: "This is the first post", user: user)
post.save

Access posts through the user:

user = User.find(1)

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

Handling Migrations

CQL migrations allow you to create and alter your database schema easily. Here are some common migration tasks:

Adding Columns:

If you need to add a new column to an existing table, generate a migration:

crystal lib/cql/bin/cql generate migration AddAgeToUsers

Update the migration to add the age column:

class AddAgeToUsers < CQL::Migration
  def up
    schema.table :users do
      add_column :age, Int32
    end
  end

  def down
    schema.table :users do
      drop_column :age
    end
  end
end

Rolling Back Migrations:
If something goes wrong with a migration, you can roll it back using:
```
AcmeDB.migrator.rollback
```
This will undo the last migration that was applied.

Conclusion

This guide has provided a basic overview of using CQL with Crystal to define records (models), create relationships, and handle migrations. You've learned how to:

Set up CQL and connect it to a database.
Create and run migrations to define your schema.
Define records and establish relationships using has_many and belongs_to.
Manage your database schema with migrations.

With this foundation, you can now expand your models, add validations, and explore more advanced querying and relationships in CQL. In the following guide, we'll take a closer look at the different relationships you can establish between models in CQL: BelongsTo, HasOne, HasMany, and ManyToMany. These relationships allow you to associate models with one another, making it easy to retrieve related data, enforce foreign key constraints, and maintain data integrity.

We'll use simple examples with CQL's DSL to help you understand how to define and use these associations effectively.

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