Rails preload, eager_load and includes

active_record_associations.rb

# Example Models

class Tournament
  attribute :name, :string
  
  has_many :teams
end
  
class Teams
  attribute :name, :string
  attribute :position, :integer
  attribute :points, :integer
  
  belongs_to :tournament
end

# preload
=begin
Preload works generating two sql querys. 
The first one is to load the model data, and the second one is to load the relationship and filter by the first model
If I want to use some filter, I should use the first model as filter, not the second one
=end

# Example of preload

Tournament.preload(:teams)

# SQL

SELECT "tournaments".* FROM "tournaments" # it will query everything (e.g. the name of tournament) of tournament
SELECT "teams".* FROM "teams" WHERE "teams"."tournament_id" IN (1) 

# eager_load
=begin
eager_load uses a JOIN, specifically LEFT OUTER JOIN (or just LEFT JOIN), 
which takes a join from the first table and the intersection with the second one
It forces one single query
=end

# Example of eager_load

Tournament.eager_load(:teams)

# SQL

SELECT "tournaments"."id" AS t0_r0, "tournaments"."name" AS t0_r1 \
FROM "tournaments" LEFT OUTER JOIN "teams" ON "teams"."tournament_id" = "tournaments"."id"

# includes
=begin
Very like the above methods, uses a single query
And accept a filter from the second table, not from the first one
=end

# Example of includes

Tournament.includes(:teams).where('team.position = 1')

#SQL 

SELECT "tournaments"."id" AS t0_r0, "tournaments"."name" AS t0_r1, "teams"."id" AS t1_r0, \
"teams"."points" AS t1_r1, \
"teams"."position" AS t1_r2, \
"teams"."tournament_id" AS t1_r3
FROM "tournaments" LEFT OUTER JOIN "teams" ON "teams"."tournament_id" = "tournaments"."id"
WHERE (team.position = 1)

# Differences and when I should use each one

=begin
They are very similar. The difference relies what kind of query I want to do and about the perfomance
each method will have for that query. preload will fires two querys, but eager_load and includes not.
And if you want to know if you should use eager_load instead of includes, use includes first and see 
the generated sql (with to_a) and the performance, if is too slow, use eager_load which always use one single query
even haves a "where"
=end