The most important concept to understand when using includes and joins is they both have their optimal use cases. Includes uses eager loading whereas joins uses lazy loading, both of which are powerful but can easily be abused to reduce or overkill performance.
If we first take a look at the Ruby on Rails documentation, the most important point made in the description of the includes method is:
With includes, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries.
In other words, when querying a table for data with an associated table, both tables are loaded into memory which in turn reduce the amount of database queries required to retrieve any associated data. In the example below we are retrieving all companies which have an associated active Person record:
@companies = Company.includes(:persons).where(:persons => { active: true } ).all
@companies.each do |company|
company.person.name
end