活动记录中的多个查询

Question

If I run this query in rails console,

def c
  ids = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  users = User.all
  members = Member.where(id: ids).includes(:locations)
  user_id_to_member_id_mapping = {} # Not empty. hash that gives member_id for each userid  
  users.each do |user|
    puts members.find_by(id: user_id_to_member_id_mapping[user.id]).location
  end
end

multiple queries (Member object queries) are fired (one for each value of i ). Are the members already in memory ( members variable)? How can I get the data needed above to make a single query?

Member Load (0.2ms)  SELECT  `members`.* FROM `members` WHERE `members`.`id` IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) AND `members`.`id` = 1 LIMIT 1
  ProfileAnswer Load (0.4ms)  SELECT `profile_answers`.* FROM `profile_answers` WHERE `profile_answers`.`member_id` = 1
  Member Load (0.4ms)  SELECT  `members`.* FROM `members` WHERE `members`.`id` IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) AND `members`.`id` = 2 LIMIT 1
  ProfileAnswer Load (0.2ms)  SELECT `profile_answers`.* FROM `profile_answers` WHERE `profile_answers`.`member_id` = 2
  Member Load (0.2ms)  SELECT  `members`.* FROM `members` WHERE `members`.`id` IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) AND `members`.`id` = 3 LIMIT 1
  ProfileAnswer Load (0.2ms)  SELECT `profile_answers`.* FROM `profile_answers` WHERE `profile_answers`.`member_id` = 3
  Member Load (0.2ms)  SELECT  `members`.* FROM `members` WHERE `members`.`id` IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) AND `members`.`id` = 4 LIMIT 1
  ProfileAnswer Load (0.2ms)  SELECT `profile_answers`.* FROM `profile_answers` WHERE `profile_answers`.`member_id` = 4
  Member Load (0.2ms)  SELECT  `members`.* FROM `members` WHERE `members`.`id` IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) AND `members`.`id` = 5 LIMIT 1
  ProfileAnswer Load (0.2ms)  SELECT `profile_answers`.* FROM `profile_answers` WHERE `profile_answers`.`member_id` = 5
  Member Load (0.4ms)  SELECT  `members`.* FROM `members` WHERE `members`.`id` IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) AND `members`.`id` = 6 LIMIT 1

I can't use member.each as the iterator is on a different condition.

Answer 1

Your code would be firing a query for each Member load because of the line

members.find_by(id: ids[i]).location

Here you have explicitly used find_by which is firing a query for each member also for location.

If reducing the query is priority then you may make use of ruby here

def c
  ids = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  users = User.all
  members = Member.where(id: ids).includes(:locations)
  user_id_to_member_id_mapping = {} # Not empty. hash that gives member_id for each userid  
  users.each do |user|
   puts members.find { |m| m.id == user_id_to_member_id_mapping[user.id] }&.location
end

Please note this would impact your performance if the Members count is very high.

Answer 2

By your code, it is supposed that you have model Location with table name 'profile_answers' table.

Member has many profile_answers & has many locations through profile_answers .

Then,

members = Members.includes(profile_answers: :locations).where(id: ids)

Answer 3

members is a scope. members.find_by executes a query on scope. Hence an N+1 .

You want instead:

members =
  Member.
    where(id: ids).
    includes(:locations).
    to_a # greedy load

puts members.map(&:location) # or whatever

Answer 4

If the location is a model, you should use includes; includes will run a query to get your initial model 's data and the included models

program = Program.first
ids = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
members = Members.includes(:locations).where(id: ids)

Now when you get the first members, calling its location will be read from the memory.

If the location is a column and you want to load all members, you can do:

program = Program.first
ids = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
members = Members.where(id: ids)
locations = members.pluck(:location)

And finally, if you don't want to use the members, you can do:

program = Program.first
ids = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
locations = Members.where(id: ids).pluck(:location)