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Type: Bug
Resolution: Duplicate
Priority: Unknown
Fix Version/s: None
Affects Version/s: None
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Ruby Drivers

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1. What would you like to communicate to the user about this feature? 2. Would you like the user to see examples of the syntax and/or executable code and its output? 3. Which versions of the driver/connector does this apply to?

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When includes is called on a relation involving an inherited class, Mongoid 8 performs additional queries when accessing the related objects. This behavior can lead to inefficiencies, as demonstrated in the code example below.

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'pry'
  gem 'pry-byebug'

  gem 'mongoid', '~> 8.0.8'
  # gem 'mongoid', '~> 7.5.4'
end

Mongoid.configure do |config|
  config.clients.default = {
    hosts: ['172.18.0.4:27017'],
    database: 'mongoid_test',
  }
end
Mongoid.purge!

class Channel
  include Mongoid::Document

  has_many :tasks
end

class Task
  include Mongoid::Document

  belongs_to :channel
end

class ProcessingTask < Task
end

3.times { ProcessingTask.create!(channel: Channel.create!) }

Mongo::Logger.logger = Logger.new($stdout)
Mongo::Logger.logger.level = Logger::DEBUG

# Observations with Mongoid versions:
# Mongoid 7.5.4: 2 queries
# Mongoid 8.0.8: 5 queries
ProcessingTask.includes(:channel).each do |task|
  pp task.channel&.id
end

# Consistent behavior (2 queries) across Mongoid versions
Task.where(_type: 'ProcessingTask').includes(:channel).each do |task|
  pp task.channel&.id
end

Example Output with Mongoid 8.0.8

When Calling `ProcessingTask.includes(:channel)`:

The output shows that Mongoid performs multiple queries for the channel relation of ProcessingTask. Specifically, each ProcessingTask access results in a separate query to fetch the associated channel.

D, [2025-01-15T17:12:03.233685 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:14 conn:1:1 sconn:280 | mongoid_test.find | STARTED | {"find"=>"tasks", "filter"=>{"_type"=>"ProcessingTask"}, "$db"=>"mongoid_test", "lsid"=>{"id"=><BSON::Binary:0x2780 type=uuid data=0x64c54f1c5b454fb
9...>}}
D, [2025-01-15T17:12:03.234118 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:14 | mongoid_test.find | SUCCEEDED | 0.000s
D, [2025-01-15T17:12:03.234902 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:15 conn:1:1 sconn:280 | mongoid_test.find | STARTED | {"find"=>"channels", "filter"=>{"_id"=>BSON::ObjectId('6787de53797b7405357df0f1')}, "limit"=>1, "$db"=>"mongoid_test", "lsid"=>{"id"=><BSON::Binary:0x2780 type=uuid data=0x64c54f1c5b454fb9...>}}
D, [2025-01-15T17:12:03.235129 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:15 | mongoid_test.find | SUCCEEDED | 0.000s
BSON::ObjectId('6787de53797b7405357df0f1')
D, [2025-01-15T17:12:03.235917 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:16 conn:1:1 sconn:280 | mongoid_test.find | STARTED | {"find"=>"channels", "filter"=>{"_id"=>BSON::ObjectId('6787de53797b7405357df0f3')}, "limit"=>1, "$db"=>"mongoid_test", "lsid"=>{"id"=><BSON::Binary:0x2780 type=uuid data=0x64c54f1c5b454fb9...>}}
D, [2025-01-15T17:12:03.236150 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:16 | mongoid_test.find | SUCCEEDED | 0.000s
BSON::ObjectId('6787de53797b7405357df0f3')
D, [2025-01-15T17:12:03.236834 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:17 conn:1:1 sconn:280 | mongoid_test.find | STARTED | {"find"=>"channels", "filter"=>{"_id"=>BSON::ObjectId('6787de53797b7405357df0f5')}, "limit"=>1, "$db"=>"mongoid_test", "lsid"=>{"id"=><BSON::Binary:0x2780 type=uuid data=0x64c54f1c5b454fb9...>}}
D, [2025-01-15T17:12:03.236990 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:17 | mongoid_test.find | SUCCEEDED | 0.000s
BSON::ObjectId('6787de53797b7405357df0f5')

When Using `Task.where(_type: 'ProcessingTask').includes(:channel)`:

In contrast, filtering explicitly by _type produces consistent query behavior across versions. Mongoid executes fewer queries by preloading all necessary relations in a single query.

D, [2025-01-15T17:12:03.237761 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:18 conn:1:1 sconn:280 | mongoid_test.find | STARTED | {"find"=>"tasks", "filter"=>{"_type"=>"ProcessingTask"}, "$db"=>"mongoid_test", "lsid"=>{"id"=><BSON::Binary:0x2780 type=uuid data=0x64c54f1c5b454fb
9...>}}
D, [2025-01-15T17:12:03.237929 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:18 | mongoid_test.find | SUCCEEDED | 0.000s
D, [2025-01-15T17:12:03.238399 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:19 conn:1:1 sconn:280 | mongoid_test.find | STARTED | {"find"=>"channels", "filter"=>{"_id"=>{"$in"=>[BSON::ObjectId('6787de53797b7405357df0f1'), BSON::ObjectId('6787de53797b7405357df0f3'), BSON::ObjectId('6787de53797b7405357df0f5')]}}, "$db"=>"mongoid_test", "lsid"=>{"id"=><BSON::Binary:0x2780 type=uuid...
D, [2025-01-15T17:12:03.238609 #1333] DEBUG -- : MONGODB | 172.18.0.4:27017 req:19 | mongoid_test.find | SUCCEEDED | 0.000s
BSON::ObjectId('6787de53797b7405357df0f1')
BSON::ObjectId('6787de53797b7405357df0f3')
BSON::ObjectId('6787de53797b7405357df0f5')

Key Takeaways

Using includes directly with an inherited class like ProcessingTask can lead to additional queries when accessing related objects (channel).
Filtering by _type explicitly (e.g., Task.where(_type: 'ProcessingTask')) ensures efficient preloading of relations.
The behavior has changed between Mongoid versions (7.5.4 vs 8.0.8), with the latter performing more queries for the same operation.

is depended on by

MONGOID-5839 Eager loading does not work for belongs_to with STI

Closed

Assignee:: Unassigned

Reporter:: Guirec Corbel

Votes:: 0 Vote for this issue

Watchers:: 3 Start watching this issue

Created:: Jan 15 2025 04:24:31 PM UTC

Updated:: Jan 15 2025 05:16:17 PM UTC

Resolved:: Jan 15 2025 04:31:57 PM UTC

GA Target Date:: None

Public Preview Target Date:: None

Private Preview Target Date:: None

Experiment Target Date:: None

Details

Description

Example Output with Mongoid 8.0.8

When Calling ProcessingTask.includes(:channel):

When Using Task.where(_type: 'ProcessingTask').includes(:channel):

Key Takeaways

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When Calling `ProcessingTask.includes(:channel)`:

When Using `Task.where(_type: 'ProcessingTask').includes(:channel)`: