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Type: Task
Resolution: Done
Priority: Major - P3
Fix Version/s: 5.0.2
Affects Version/s: None
Component/s: None
Labels:
None

If you try to use has_and_belongs_to_many eager loading with dependent objects having _id field of type BSON::Binary you get empty arrays instead of children. Mongoid does perform a single query to load all these objects, but then doesn't seem to be able to find results and performs additional queries with nil instead of actual children IDs. This seems to be something specific to Binary fields, because IDs of type String work just as well as regular ObjectId type.

Test script:

#gem "mongoid", "=3.1.6"
gem "mongoid", "=4.0.0"

require "mongoid"

puts "Using Mongoid #{Mongoid::VERSION}"

Mongoid.configure {|c| c.connect_to "test"}

def verbose
  old_logger = Moped.logger
  Moped.logger = Logger.new(STDOUT)
  puts "\n\n"
  yield
ensure
  Moped.logger = old_logger
end

class Parent
  include Mongoid::Document
  has_and_belongs_to_many :children, inverse_of: nil
end

class Child
  include Mongoid::Document
  field :_id, type: BSON::Binary, default: -> {
    BSON::Binary.new SecureRandom.hex(3)
  }
  field :something, type: Integer, default: -> {rand(1_000)}
end

Parent.delete_all
3.times do
  parent = Parent.create
  5.times do
    parent.children << Child.create
  end
end

verbose do
  puts "Without eager loading"
  Parent.all.each {|p| puts p.children.map(&:something).inspect}
end

verbose do
  puts "With eager loading"
  Parent.all.includes(:children).each {|p| puts p.children.map(&:something).inspect}
end

Output:

mongoid-errors$ruby eager_load.rb 
Using Mongoid 4.0.0
W, [2014-08-24T02:22:12.058385 #95340]  WARN -- : Overwriting existing field _id in class Child.


Without eager loading
D, [2014-08-24T02:22:12.088171 #95340] DEBUG -- :   MOPED: 127.0.0.1:27017 QUERY        database=test collection=parents selector={} flags=[] limit=0 skip=0 batch_size=nil fields=nil runtime: 0.3770ms
D, [2014-08-24T02:22:12.089165 #95340] DEBUG -- :   MOPED: 127.0.0.1:27017 QUERY        database=test collection=children selector={"$and"=>[{"_id"=>{"$in"=>[#<BSON::Binary:0x007f7f7aa88e90 @data="64d53e", @type=:generic>, #<BSON::Binary:0x007f7f7aa88c88 @data="6ed747", @type=:generic>, #<BSON::Binary:0x007f7f7aa888f0 @data="be81ff", @type=:generic>, #<BSON::Binary:0x007f7f7aa88710 @data="e950c4", @type=:generic>, #<BSON::Binary:0x007f7f7aa88580 @data="7da04e", @type=:generic>]}}]} flags=[] limit=0 skip=0 batch_size=nil fields=nil runtime: 0.4480ms
[201, 753, 564, 497, 433]
D, [2014-08-24T02:22:12.090226 #95340] DEBUG -- :   MOPED: 127.0.0.1:27017 QUERY        database=test collection=children selector={"$and"=>[{"_id"=>{"$in"=>[#<BSON::Binary:0x007f7f7aa88008 @data="56fe72", @type=:generic>, #<BSON::Binary:0x007f7f7aa7be98 @data="9a6cdf", @type=:generic>, #<BSON::Binary:0x007f7f7aa7bd58 @data="96956e", @type=:generic>, #<BSON::Binary:0x007f7f7aa7bc40 @data="c8e19d", @type=:generic>, #<BSON::Binary:0x007f7f7aa7ba38 @data="7e7f2e", @type=:generic>]}}]} flags=[] limit=0 skip=0 batch_size=nil fields=nil runtime: 0.4310ms
[21, 167, 84, 450, 195]
D, [2014-08-24T02:22:12.091216 #95340] DEBUG -- :   MOPED: 127.0.0.1:27017 QUERY        database=test collection=children selector={"$and"=>[{"_id"=>{"$in"=>[#<BSON::Binary:0x007f7f7aa7b5b0 @data="0b98dd", @type=:generic>, #<BSON::Binary:0x007f7f7aa7b470 @data="32c896", @type=:generic>, #<BSON::Binary:0x007f7f7aa7b358 @data="5b0738", @type=:generic>, #<BSON::Binary:0x007f7f7aa7b218 @data="693c6d", @type=:generic>, #<BSON::Binary:0x007f7f7aa7b060 @data="4d2996", @type=:generic>]}}]} flags=[] limit=0 skip=0 batch_size=nil fields=nil runtime: 0.4080ms
[845, 431, 694, 885, 190]


With eager loading
D, [2014-08-24T02:22:12.092050 #95340] DEBUG -- :   MOPED: 127.0.0.1:27017 QUERY        database=test collection=parents selector={} flags=[] limit=0 skip=0 batch_size=nil fields=nil runtime: 0.3240ms
D, [2014-08-24T02:22:12.093532 #95340] DEBUG -- :   MOPED: 127.0.0.1:27017 QUERY        database=test collection=children selector={"_id"=>{"$in"=>[#<BSON::Binary:0x007f7f7ab53a78 @data="64d53e", @type=:generic>, #<BSON::Binary:0x007f7f7ab53960 @data="6ed747", @type=:generic>, #<BSON::Binary:0x007f7f7ab53870 @data="be81ff", @type=:generic>, #<BSON::Binary:0x007f7f7ab53730 @data="e950c4", @type=:generic>, #<BSON::Binary:0x007f7f7ab53618 @data="7da04e", @type=:generic>, #<BSON::Binary:0x007f7f7ab53190 @data="56fe72", @type=:generic>, #<BSON::Binary:0x007f7f7ab53050 @data="9a6cdf", @type=:generic>, #<BSON::Binary:0x007f7f7ab52f10 @data="96956e", @type=:generic>, #<BSON::Binary:0x007f7f7ab52e20 @data="c8e19d", @type=:generic>, #<BSON::Binary:0x007f7f7ab52c90 @data="7e7f2e", @type=:generic>, #<BSON::Binary:0x007f7f7ab527e0 @data="0b98dd", @type=:generic>, #<BSON::Binary:0x007f7f7ab526f0 @data="32c896", @type=:generic>, #<BSON::Binary:0x007f7f7ab52600 @data="5b0738", @type=:generic>, #<BSON::Binary:0x007f7f7ab524e8 @data="693c6d", @type=:generic>, #<BSON::Binary:0x007f7f7ab523d0 @data="4d2996", @type=:generic>]}} flags=[] limit=0 skip=0 batch_size=nil fields=nil runtime: 0.6100ms
D, [2014-08-24T02:22:12.094905 #95340] DEBUG -- :   MOPED: 127.0.0.1:27017 QUERY        database=test collection=children selector={"$and"=>[{"_id"=>{"$in"=>[nil, nil, nil, nil, nil]}}]} flags=[] limit=0 skip=0 batch_size=nil fields=nil runtime: 0.3530ms
[]
D, [2014-08-24T02:22:12.095608 #95340] DEBUG -- :   MOPED: 127.0.0.1:27017 QUERY        database=test collection=children selector={"$and"=>[{"_id"=>{"$in"=>[nil, nil, nil, nil, nil]}}]} flags=[] limit=0 skip=0 batch_size=nil fields=nil runtime: 0.3270ms
[]
D, [2014-08-24T02:22:12.096201 #95340] DEBUG -- :   MOPED: 127.0.0.1:27017 QUERY        database=test collection=children selector={"$and"=>[{"_id"=>{"$in"=>[nil, nil, nil, nil, nil]}}]} flags=[] limit=0 skip=0 batch_size=nil fields=nil runtime: 0.2800ms
[]

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