Eager Loading vs. Join Queries in Ruby on Rails: Understanding `includes` and `joins`

• Fetches data from all related tables in a single query.
• This is efficient when you need to access data from the associated tables for most or all of the results.
• It avoids the N+1 query problem, where you would fire a separate query for each record to fetch linked data.

joins (Join Queries):

• Adds a JOIN clause to the SQL query, allowing you to filter or perform operations on the joined data.
• You only retrieve data from the tables you specify in the join.
• This is useful when you need to filter the results based on data from the joined tables or only need data from a specific related table.

Here's a table summarizing the key differences:

Choosing between includes and joins:

• Use includes when you need the data from related tables for most results and want to optimize performance.
• Use joins when you need to filter the data based on the joined tables or only need specific data from a related table.

# Models (assuming User has a belongs_to association with Company)

class User < ApplicationRecord
  belongs_to :company
end

class Company < ApplicationRecord
  has_many :users
end

# Fetching users with their company data in one query
users = User.includes(:company)

# Now you can access user.company.name without additional queries
users.each do |user|
  puts user.name + " works at " + user.company.name
end

Example 2: Using joins

# Fetching users who work in a specific company (using join)
specific_company_id = 1
users = User.joins(:company).where(companies: { id: specific_company_id })

# This query only retrieves users with company_id equal to specific_company_id
users.each do |user|
  puts user.name
end

These are just basic examples. In practice, you might use more complex joins with conditions or multiple associations with includes.

1. Preloading:

   users = User.preload(:company => :name)  # Load only company name
   

2. Separate Queries:

   This isn't ideal for performance, but it's an option if :includes or :joins don't fit your specific needs. You can write separate queries to fetch the main data and then loop through the results to fetch data from related tables using their primary keys.

   users = User.all
   users.each do |user|
     company = Company.find(user.company_id)
     puts user.name + " works at " + company.name
   end
   

Choosing the Right Method:

• Use :includes for most scenarios where you need data from related models for all or most results. It offers good performance and avoids N+1 queries.
• Consider :preload if you only need specific data from related models and want to optimize memory usage compared to :includes.
• Use separate queries only as a last resort if :includes and :joins are too complex for your specific case, but be aware of the performance implications.