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MySQL to ActiveRecord Converter Guide

Complete guide for converting MySQL queries to Rails ActiveRecord with MySQL-specific patterns

What you'll learn:

  • • Convert MySQL-specific syntax to ActiveRecord
  • • Handle MySQL data types and functions
  • • Optimize MySQL queries in Rails applications
  • • Work with MySQL-specific features like full-text search

Table of Contents

1. MySQL Setup in Rails

Database Configuration

Gemfile Setup

# Gemfile
gem 'mysql2', '~> 0.5'

Database Configuration

# config/database.yml
default: &default
  adapter: mysql2
  encoding: utf8mb4
  pool: 5
  username: root
  password: 
  socket: /tmp/mysql.sock

development:
  <<: *default
  database: myapp_development

test:
  <<: *default
  database: myapp_test

production:
  <<: *default
  database: myapp_production
  username: myapp
  password: 
  host:

MySQL-Specific Configuration

# config/database.yml - Additional MySQL options
default: &default
  adapter: mysql2
  encoding: utf8mb4
  collation: utf8mb4_unicode_ci
  pool: 5
  reconnect: false
  strict: true  # Enable strict mode
  variables:
    sql_mode: STRICT_TRANS_TABLES,NO_ZERO_DATE,NO_ZERO_IN_DATE,ERROR_FOR_DIVISION_BY_ZERO

2. Basic Query Conversion

Common MySQL to ActiveRecord Patterns

LIMIT and OFFSET

MySQL SQL:

SELECT * FROM users 
ORDER BY created_at DESC 
LIMIT 10 OFFSET 20;

Rails ActiveRecord:

User.order(created_at: :desc)
    .limit(10)
    .offset(20)

# Or using page-based pagination
User.order(created_at: :desc)
    .limit(10)
    .offset(page * 10)

CASE Statements

MySQL SQL:

SELECT *,
  CASE 
    WHEN age < 18 THEN 'Minor'
    WHEN age < 65 THEN 'Adult'
    ELSE 'Senior'
  END as age_group
FROM users;

Rails ActiveRecord:

User.select("*,
  CASE 
    WHEN age < 18 THEN 'Minor'
    WHEN age < 65 THEN 'Adult'
    ELSE 'Senior'
  END as age_group")

Date and Time Functions

MySQL SQL:

SELECT * FROM orders 
WHERE DATE(created_at) = CURDATE();

SELECT * FROM users 
WHERE YEAR(created_at) = 2024;

Rails ActiveRecord:

Order.where('DATE(created_at) = CURDATE()')

# Better: Use Rails date handling
Order.where(created_at: Date.current.all_day)

User.where('YEAR(created_at) = ?', 2024)
# Better: Use Rails date range
User.where(created_at: Date.new(2024).all_year)

3. MySQL-Specific Functions

String Functions

CONCAT and String Manipulation

MySQL SQL:

SELECT CONCAT(first_name, ' ', last_name) as full_name
FROM users;

SELECT SUBSTRING(email, 1, LOCATE('@', email) - 1) as username
FROM users;

Rails ActiveRecord:

User.select("CONCAT(first_name, ' ', last_name) as full_name")

User.select("SUBSTRING(email, 1, LOCATE('@', email) - 1) as username")

# Or define in model
class User < ApplicationRecord
  def full_name
    "#{first_name} #{last_name}"
  end
end

REGEXP and Pattern Matching

MySQL SQL:

SELECT * FROM users 
WHERE email REGEXP '^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$';

SELECT * FROM products 
WHERE name REGEXP '(phone|mobile|cell)';

Rails ActiveRecord:

User.where('email REGEXP ?', '^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$')

Product.where('name REGEXP ?', '(phone|mobile|cell)')

# Using Rails validations is often better
class User < ApplicationRecord
  validates :email, format: { with: URI::MailTo::EMAIL_REGEXP }
end

Numeric and Aggregate Functions

Mathematical Functions

MySQL SQL:

SELECT 
  ROUND(price, 2) as rounded_price,
  CEIL(price) as ceiling_price,
  FLOOR(price) as floor_price
FROM products;

SELECT ABS(profit_loss) as absolute_value
FROM financial_records;

Rails ActiveRecord:

Product.select('
  ROUND(price, 2) as rounded_price,
  CEIL(price) as ceiling_price,
  FLOOR(price) as floor_price
')

FinancialRecord.select('ABS(profit_loss) as absolute_value')

GROUP_CONCAT

MySQL SQL:

SELECT 
  user_id,
  GROUP_CONCAT(tag_name SEPARATOR ', ') as tags
FROM user_tags 
GROUP BY user_id;

Rails ActiveRecord:

UserTag.group(:user_id)
       .select("user_id, GROUP_CONCAT(tag_name SEPARATOR ', ') as tags")

# Alternative with Rails associations
User.joins(:tags).group(:id)
    .select('users.*, GROUP_CONCAT(tags.name SEPARATOR ", ") as tag_list')

4. Data Types and Storage

MySQL Data Types in Rails

Text and Binary Types

# Migration with MySQL-specific types
class CreatePostsWithMysqlTypes < ActiveRecord::Migration[7.0]
  def change
    create_table :posts do |t|
      t.string :title, limit: 255        # VARCHAR(255)
      t.text :content, limit: 65535      # TEXT
      t.text :long_content, limit: 4294967295  # LONGTEXT
      t.binary :thumbnail, limit: 65535  # BLOB
      t.json :metadata                   # JSON (MySQL 5.7+)
      
      t.timestamps
    end
  end
end

Numeric Types

# Migration with MySQL numeric types
class CreateProductsWithMysqlTypes < ActiveRecord::Migration[7.0]
  def change
    create_table :products do |t|
      t.integer :stock, limit: 1        # TINYINT
      t.integer :views, limit: 2        # SMALLINT
      t.integer :sales, limit: 3        # MEDIUMINT
      t.integer :total_orders, limit: 4 # INT
      t.integer :big_number, limit: 8   # BIGINT
      
      t.decimal :price, precision: 10, scale: 2
      t.float :rating, limit: 24        # FLOAT
      t.float :score, limit: 53         # DOUBLE
      
      t.timestamps
    end
  end
end

JSON Support (MySQL 5.7+)

MySQL SQL:

SELECT * FROM users 
WHERE JSON_EXTRACT(preferences, '$.theme') = 'dark';

SELECT JSON_UNQUOTE(JSON_EXTRACT(data, '$.name')) as name
FROM products;

Rails ActiveRecord:

User.where("JSON_EXTRACT(preferences, '$.theme') = ?", 'dark')

Product.select("JSON_UNQUOTE(JSON_EXTRACT(data, '$.name')) as name")

# Rails automatically handles JSON serialization
user = User.create(preferences: { theme: 'dark', lang: 'en' })
user.preferences['theme'] # => 'dark'

5. Full-Text Search

MySQL FULLTEXT Search

Setting up FULLTEXT Index

# Migration for FULLTEXT index
class AddFulltextIndexToPosts < ActiveRecord::Migration[7.0]
  def up
    execute "ALTER TABLE posts ADD FULLTEXT(title, content)"
  end
  
  def down
    execute "ALTER TABLE posts DROP INDEX title"
  end
end

Basic FULLTEXT Search

MySQL SQL:

SELECT *, MATCH(title, content) AGAINST('rails activerecord') as relevance
FROM posts 
WHERE MATCH(title, content) AGAINST('rails activerecord')
ORDER BY relevance DESC;

Rails ActiveRecord:

search_term = 'rails activerecord'
Post.select("*, MATCH(title, content) AGAINST(?) as relevance", search_term)
    .where("MATCH(title, content) AGAINST(?)", search_term)
    .order('relevance DESC')

Boolean and Natural Language Search

MySQL SQL:

-- Boolean mode search
SELECT * FROM posts 
WHERE MATCH(title, content) AGAINST('+rails -php' IN BOOLEAN MODE);

-- Natural language search
SELECT * FROM posts 
WHERE MATCH(title, content) AGAINST('database optimization' IN NATURAL LANGUAGE MODE);

Rails ActiveRecord:

# Boolean mode search
Post.where("MATCH(title, content) AGAINST(? IN BOOLEAN MODE)", '+rails -php')

# Natural language search
Post.where("MATCH(title, content) AGAINST(? IN NATURAL LANGUAGE MODE)", 'database optimization')

# Scope for reusability
class Post < ApplicationRecord
  scope :fulltext_search, ->(query) { where("MATCH(title, content) AGAINST(?)", query) }
end

6. JOINs and Subqueries

Complex JOIN Patterns

Multiple Table JOINs

MySQL SQL:

SELECT 
  u.name, 
  p.title, 
  c.name as category_name,
  COUNT(co.id) as comment_count
FROM users u
JOIN posts p ON p.user_id = u.id
JOIN categories c ON p.category_id = c.id
LEFT JOIN comments co ON co.post_id = p.id
GROUP BY u.id, p.id, c.id;

Rails ActiveRecord:

User.joins(posts: [:category])
    .left_joins(posts: :comments)
    .select('users.name, posts.title, categories.name as category_name, COUNT(comments.id) as comment_count')
    .group('users.id, posts.id, categories.id')

Subqueries with EXISTS

MySQL SQL:

SELECT * FROM users u
WHERE EXISTS (
  SELECT 1 FROM posts p 
  WHERE p.user_id = u.id 
  AND p.published = 1
);

Rails ActiveRecord:

User.where('EXISTS (?)', 
  Post.where('posts.user_id = users.id')
      .where(published: true)
      .select('1')
)

# Or using joins (often more efficient)
User.joins(:posts).where(posts: { published: true }).distinct

Correlated Subqueries

MySQL SQL:

SELECT *,
  (SELECT COUNT(*) FROM posts WHERE user_id = users.id) as post_count,
  (SELECT MAX(created_at) FROM posts WHERE user_id = users.id) as last_post_date
FROM users;

Rails ActiveRecord:

User.select('users.*,
  (SELECT COUNT(*) FROM posts WHERE user_id = users.id) as post_count,
  (SELECT MAX(created_at) FROM posts WHERE user_id = users.id) as last_post_date')

# Better: Use includes to prevent N+1
User.left_joins(:posts)
    .select('users.*, COUNT(posts.id) as post_count, MAX(posts.created_at) as last_post_date')
    .group('users.id')

7. Performance Optimization

MySQL-Specific Optimizations

Index Strategy

# Migration with MySQL-optimized indexes
class OptimizeMysqlIndexes < ActiveRecord::Migration[7.0]
  def change
    # Composite index for common query patterns
    add_index :posts, [:user_id, :published, :created_at]
    
    # Covering index to avoid table lookups
    add_index :users, [:email], name: 'index_users_on_email_covering'
    
    # Partial index for active records only
    add_index :users, :created_at, where: "active = 1"
    
    # FULLTEXT index for search
    execute "ALTER TABLE posts ADD FULLTEXT(title, content)"
  end
end

Query Optimization

# Use EXPLAIN to analyze queries
User.where(active: true).explain

# Force index usage when needed
User.from('users FORCE INDEX (index_users_on_email)')
    .where(email: 'user@example.com')

# Use SQL_CALC_FOUND_ROWS for pagination (MySQL specific)
ActiveRecord::Base.connection.execute('SELECT SQL_CALC_FOUND_ROWS * FROM users LIMIT 10')
total_count = ActiveRecord::Base.connection.execute('SELECT FOUND_ROWS()').first[0]

MySQL Performance Tips

  • • Use composite indexes for multi-column WHERE clauses
  • • Consider query cache settings for read-heavy applications
  • • Use LIMIT to prevent large result sets
  • • Optimize MyISAM vs InnoDB storage engine choice
  • • Use connection pooling for concurrent requests
  • • Consider read replicas for scaling read operations

8. Migration Strategies

Step-by-Step Migration Process

1. Audit Existing MySQL Queries

# Find MySQL-specific syntax in your codebase
grep -r "LIMIT.*OFFSET\|GROUP_CONCAT\|REGEXP\|FULLTEXT" app/
grep -r "FORCE INDEX\|USE INDEX\|IGNORE INDEX" app/

2. Convert Common Patterns First

# Start with simple conversions
# MySQL: SELECT * FROM users WHERE email LIKE '%@gmail.com' LIMIT 10
User.where('email LIKE ?', '%@gmail.com').limit(10)

# Then move to complex queries
# MySQL: SELECT u.*, COUNT(p.id) as post_count FROM users u LEFT JOIN posts p ON p.user_id = u.id GROUP BY u.id
User.left_joins(:posts).group(:id).select('users.*, COUNT(posts.id) as post_count')

3. Handle MySQL-Specific Features

# Create helper methods for MySQL-specific functions
class ApplicationRecord < ActiveRecord::Base
  scope :fulltext_search, ->(query) do
    where("MATCH(title, content) AGAINST(?)", query) if query.present?
  end
  
  def self.group_concat(column, separator = ',')
    select("GROUP_CONCAT(#{column} SEPARATOR '#{separator}') as concatenated")
  end
end

9. Common Conversion Patterns

Frequently Used MySQL to ActiveRecord Conversions

Pagination with Total Count

MySQL SQL:

SELECT SQL_CALC_FOUND_ROWS * 
FROM users 
WHERE active = 1 
ORDER BY created_at DESC 
LIMIT 10 OFFSET 0;

SELECT FOUND_ROWS() as total;

Rails ActiveRecord:

# Using Kaminari gem
User.where(active: true)
    .order(created_at: :desc)
    .page(1)
    .per(10)

# Manual pagination
users = User.where(active: true)
            .order(created_at: :desc)
            .limit(10)
            .offset(0)
total = User.where(active: true).count

Dynamic WHERE Clauses

MySQL SQL:

SELECT * FROM products 
WHERE 1=1
  AND (category_id = ? OR ? IS NULL)
  AND (price BETWEEN ? AND ? OR ? IS NULL)
  AND (name LIKE ? OR ? IS NULL);

Rails ActiveRecord:

products = Product.all
products = products.where(category_id: category_id) if category_id.present?
products = products.where(price: min_price..max_price) if min_price && max_price
products = products.where('name LIKE ?', "%#{name}%") if name.present?

# Using scopes
class Product < ApplicationRecord
  scope :by_category, ->(cat) { where(category_id: cat) if cat.present? }
  scope :by_price_range, ->(min, max) { where(price: min..max) if min && max }
  scope :by_name, ->(name) { where('name LIKE ?', "%#{name}%") if name.present? }
end

Ranking and Top N Queries

MySQL SQL:

SELECT 
  u.*,
  @rank := @rank + 1 as rank
FROM users u
CROSS JOIN (SELECT @rank := 0) r
ORDER BY score DESC
LIMIT 10;

Rails ActiveRecord:

# Using ROW_NUMBER() (MySQL 8.0+)
User.select('*, ROW_NUMBER() OVER (ORDER BY score DESC) as rank')
    .order(score: :desc)
    .limit(10)

# For older MySQL versions
users = User.order(score: :desc).limit(10)
users.each_with_index { |user, index| user.rank = index + 1 }

Migration Checklist

  • • ✅ Update database.yml to use mysql2 adapter
  • • ✅ Convert MySQL-specific SQL to ActiveRecord methods
  • • ✅ Update FULLTEXT search queries
  • • ✅ Optimize indexes for ActiveRecord query patterns
  • • ✅ Test performance with realistic data volumes
  • • ✅ Update any MySQL-specific configuration
  • • ✅ Consider connection pooling settings

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