Core Concepts Overview

Master the fundamentals - Essential building blocks for effective CQL development

Welcome to the Core Concepts section! This is your comprehensive guide to understanding CQL's fundamental architecture and features. Whether you're new to ORMs or transitioning from other frameworks, these concepts will give you the solid foundation needed to build robust Crystal applications with CQL.

What You'll Master

This section covers the essential building blocks that make CQL powerful and developer-friendly:

Schema Definition - Type-safe database structure design
Database Initialization - Connection setup and management
Schema Alterations - Evolving your database structure
Migrations - Systematic schema evolution and versioning
CRUD Operations - Create, Read, Update, Delete mastery
Design Patterns - Active Record, Repository, and Data Mapper approaches

Learning Path

Recommended Path:

Schema Definition → Learn how to define your database structure
Database Initialization → Set up connections and build your schema
CRUD Operations → Master basic database interactions
Schema Alterations → Understand how to modify existing structures
Migrations → Implement systematic schema evolution
Design Patterns → Choose the right architectural approach
Active Record Guide → Apply concepts to real applications

Schema Definition

Type-safe database structure design with Crystal's macro system

CQL's schema definition system leverages Crystal's powerful macro capabilities to provide compile-time safety and intuitive database structure design.

Key Features

Type Safety - Compile-time validation of schema structure
Multi-Database Support - Works with PostgreSQL, MySQL, and SQLite
Declarative DSL - Clean, readable schema definitions
Constraint Support - Foreign keys, unique constraints, and check constraints

Quick Example

# Define a complete schema with relationships
UserSchema = CQL::Schema.define(
  :user_app,
  adapter: CQL::Adapter::Postgres,
  uri: ENV["DATABASE_URL"]
) do

  table :users do
    primary :id, Int64, auto_increment: true
    column :name, String, size: 100
    column :email, String, size: 255
    column :age, Int32
    column :active, Bool, default: true
    timestamps

    # Constraints
    unique_constraint [:email]
    check_constraint "age >= 0 AND age <= 150"
  end

  table :posts do
    primary :id, Int64, auto_increment: true
    column :title, String, size: 255
    column :content, String
    column :user_id, Int64
    column :published, Bool, default: false
    timestamps

    # Foreign key relationship
    foreign_key :user_id, references: :users, on_delete: :cascade
    index [:user_id, :published]
  end
end

What You'll Learn

Table Structure - Defining columns, types, and constraints
Relationships - Foreign keys and referential integrity
Indexing Strategy - Performance optimization through proper indexing
Database-Specific Features - Leveraging unique database capabilities

👉 Learn Schema Definition →

Database Initialization

Efficient connection management and schema building

Learn how to initialize your database connections, configure adapters, and build your schema for optimal performance and reliability.

Key Concepts

Connection Pooling - Efficient database connection management
Environment Configuration - Development, testing, and production setups
Schema Building - Converting definitions to actual database tables
Error Handling - Robust connection error management

Quick Example

# Environment-specific database setup
case ENV["CRYSTAL_ENV"]?
when "production"
  MyDB = CQL::Schema.define(
    :production_db,
    adapter: CQL::Adapter::Postgres,
    uri: ENV["DATABASE_URL"],
    pool_size: 20,
    checkout_timeout: 10.seconds
  )

when "test"
  MyDB = CQL::Schema.define(
    :test_db,
    adapter: CQL::Adapter::SQLite,
    uri: "sqlite3://:memory:",  # Fast in-memory testing
    pool_size: 1
  )

else # development
  MyDB = CQL::Schema.define(
    :dev_db,
    adapter: CQL::Adapter::SQLite,
    uri: "sqlite3://./db/development.db",
    pool_size: 5
  )
end

# Build the actual database schema
MyDB.build

What You'll Learn

Adapter Configuration - Setting up PostgreSQL, MySQL, and SQLite
Connection Strategies - Pool sizing and timeout management
Environment Management - Development vs production configurations
Health Checks - Monitoring connection status

👉 Learn Database Initialization →

CRUD Operations

Master Create, Read, Update, Delete operations with type safety

CRUD operations are the foundation of database interactions. CQL provides both Active Record and Repository patterns for maximum flexibility.

Core Capabilities

Type-Safe Operations - Compile-time validation of data operations
Multiple Patterns - Active Record and Repository approaches
Advanced Querying - Complex WHERE conditions and joins
Batch Operations - Efficient bulk operations
Transaction Support - ACID compliance with automatic rollback

Quick Examples

Active Record Pattern:

# Create
user = User.create!(
  name: "Alice Johnson",
  email: "alice@example.com",
  age: 28
)

# Read
users = User.where(active: true).all
user = User.find!(user.id)

# Update
user.update!(age: 29)

# Delete
user.destroy!

Repository Pattern:

users = CQL::Repository(User, Int64).new(MyDB, :users)

# Create
user_id = users.create(name: "Bob", email: "bob@example.com")

# Read
user = users.find!(user_id)
active_users = users.where(active: true).all

# Update
users.update(user_id, age: 30)

# Delete
users.delete(user_id)

What You'll Learn

Basic CRUD - Create, read, update, delete operations
Query Building - Complex WHERE conditions and joins
Batch Operations - Efficient bulk data operations
Error Handling - Graceful error management

👉 Learn CRUD Operations →

Schema Alterations

Evolving your database structure safely and efficiently

Learn how to modify existing database schemas without losing data or causing downtime.

Key Concepts

Safe Alterations - Modify tables without data loss
Constraint Management - Add, modify, and remove constraints
Index Optimization - Improve query performance
Migration Integration - Work with CQL's migration system

Quick Example

# Alter existing table
MyDB.alter_table :users do |t|
  # Add new column
  t.add_column :phone, String, size: 20

  # Modify existing column
  t.modify_column :email, String, size: 255, null: false

  # Add constraint
  t.add_unique_constraint [:email, :phone]

  # Add index
  t.add_index [:created_at, :active]
end

What You'll Learn

Column Operations - Add, modify, and remove columns
Constraint Management - Foreign keys, unique constraints, checks
Index Strategy - Performance optimization
Data Migration - Safely transforming existing data

👉 Learn Schema Alterations →

Migrations

Version-controlled schema evolution for team development

Migrations provide a systematic way to evolve your database schema over time, ensuring consistency across all environments.

Key Features

Version Control - Track schema changes over time
Reversible Operations - Rollback changes when needed
Team Collaboration - Consistent schema across environments
Environment Management - Different schemas for dev, test, prod

Quick Example

class CreateUsers < CQL::Migration
  def up
    create_table :users do |t|
      t.string :name, null: false
      t.string :email, null: false
      t.integer :age
      t.boolean :active, default: true
      t.timestamps

      t.index :email, unique: true
    end
  end

  def down
    drop_table :users
  end
end

What You'll Learn

Migration Structure - Creating and organizing migrations
Schema Operations - Table creation, modification, and deletion
Data Migrations - Transforming existing data
Rollback Strategy - Reversing changes safely

👉 Learn Migrations →

Design Patterns

Choose the right architectural approach for your application

CQL supports multiple design patterns, allowing you to choose the approach that best fits your application's needs.

Available Patterns

Active Record - Domain-rich applications with business logic in models
Repository - Data-centric architectures with clean separation
Data Mapper - Complex domain models with flexible mapping

Pattern Comparison

Pattern

Best For

Complexity

Flexibility

Active Record

CRUD-heavy apps, rapid prototyping

Low

Medium

Repository

Complex business logic, testability

Medium

High

Data Mapper

Complex domains, multiple data sources

High

Very High

What You'll Learn

Pattern Selection - Choosing the right approach
Implementation - How to implement each pattern
Best Practices - When and how to use each pattern
Migration Between Patterns - Switching approaches as needed

👉 Learn Design Patterns →

Next Steps

Now that you understand the core concepts, you're ready to build real applications! Here's what to explore next:

Ready to Build?

Getting Started Guide - Build your first CQL application
Active Record with CQL - Learn the most popular pattern
Configuration Guide - Set up for your environment

Need More Details?

Architecture Overview - Deep dive into CQL's design
Best Practices - Follow proven patterns
Examples - See CQL in action

Getting Stuck?

Troubleshooting Guide - Solve common problems
FAQ - Find quick answers
Community - Get help from others

The concepts you've learned here form the foundation for everything else in CQL. Take your time to understand them well - they'll make the advanced topics much easier to grasp!

Try building a simple application using these concepts before moving to the advanced guides. The Examples section has great starting points!

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Last updated 4 months ago

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