rails: ActiveRecord `pluck(:name)` returning different number of results than without `pluck`

Steps to reproduce

Have a Project model with a has_many improvements. Improvements and projects have a name column.

Run these 2 queries:

Project.includes(:improvements).where(improvements: { name: 'one' })
Project.includes(:improvements).where(improvements: { name: 'one' }).pluck(:name)

Expected behavior

I expect each query to return the same number of results (i.e. running #count on them will return the same number)

Actual behavior

The query with the pluck(:name) in it returns a different result set with many duplicates.

Without a pluck it runs this query:

SELECT DISTINCT "projects"."id" FROM "projects" LEFT OUTER JOIN "improvements" ON "improvements"."project_id" = "projects"."id" WHERE "improvements"."name" = $1 LIMIT $2

Followed by another long query to get all the actual data. However, thanks to the DISTINCT restriction, no duplicate data is returned.

With the pluck(:name) method, I get this query:

 SELECT "projects"."name" FROM "projects" LEFT OUTER JOIN "improvements" ON "improvements"."project_id" = "projects"."id" WHERE "improvements"."name" = $1 

Without a second query since no other data is required. However, because this is no longer using DISTINCT on the project ID, I am getting multiple results, even though I only have one project.

In my case I have a single project called “hello” with 5 improvements. 4 of which have the name “one” I expect to get an array equal to ["hello"], but instead I get ["hello", "hello", "hello", "hello"].

I understand WHY this is happening and it makes sense, but the expected behaviour from pluck(:name) is that it simply gets a single column from the query results, and does not alter the query itself. In this case it is completely changing what the query does.

Note: A temporary solution I am using is to convert the result set to an array and then plucking the results

System configuration

Rails version: Rails 6.0.3.2 Ruby version: ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux]

Reproducible script

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "6.0.3"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
    t.string :name
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
    t.string :name
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_pluck_does_not_effect_results
    post = Post.create!(name: 'hello')
    post.comments << Comment.create!(name: 'one')
    post.comments << Comment.create!(name: 'one')
    post.comments << Comment.create!(name: 'one')
    post.comments << Comment.create!(name: 'one')

    without_pluck = Post.includes(:comments).where(comments: { name: 'one' }).to_a.count
    with_pluck = Post.includes(:comments).where(comments: { name: 'one' }).pluck(:name).to_a.count

    assert_equal without_pluck, with_pluck
  end
end