Don't over protect the MySQL users on insert_all

MySQL don't support conflict target on insert_all so we can't pass the
:unique_by option. But, if you don't check if the index exists MySQL will
successfully inset the data.

If there is a conflict in an index like the primary key the database will
raise an error. We should let the database decide if the data is acceptable
or not.

The test added on this commit exercise a table that has composite primary
keys, but, in order to keep all the Rails behavior working (given Rails
doesn't officially support composite primary keys) we set the primary
key to an auto increment :id column.

activerecord/lib/active_record/insert_all.rb

@@ -69,6 +69,8 @@ module ActiveRecord
       attr_reader :scope_attributes
 
       def find_unique_index_for(unique_by)
+        return unique_by if !connection.supports_insert_conflict_target?
+
         name_or_columns = unique_by || model.primary_key
         match = Array(name_or_columns).map(&:to_s)
 

activerecord/test/cases/insert_all_test.rb

@@ -3,6 +3,7 @@
 require "cases/helper"
 require "models/author"
 require "models/book"
+require "models/cart"
 require "models/speedometer"
 require "models/subscription"
 require "models/subscriber"
@@ -208,6 +209,33 @@ class InsertAllTest < ActiveRecord::TestCase
     end
   end
 
+  def test_insert_all_and_upsert_all_works_with_composite_primary_keys_when_unique_by_is_provided
+    skip unless supports_insert_conflict_target?
+
+    assert_difference "Cart.count", 2 do
+      Cart.insert_all [{ id: 1, shop_id: 1, title: "My cart" }], unique_by: [:shop_id, :id]
+
+      Cart.upsert_all [{ id: 3, shop_id: 2, title: "My other cart" }], unique_by: [:shop_id, :id]
+    end
+
+    error = assert_raises ArgumentError do
+      Cart.insert_all! [{ id: 2, shop_id: 1, title: "My cart" }]
+    end
+    assert_match "No unique index found for id", error.message
+  end
+
+  def test_insert_all_and_upsert_all_works_with_composite_primary_keys_when_unique_by_is_not_provided
+    skip unless supports_insert_on_duplicate_skip? && !supports_insert_conflict_target?
+
+    assert_difference "Cart.count", 3 do
+      Cart.insert_all [{ id: 1, shop_id: 1, title: "My cart" }]
+
+      Cart.insert_all! [{ id: 2, shop_id: 1, title: "My cart 2" }]
+
+      Cart.upsert_all [{ id: 3, shop_id: 2, title: "My other cart" }]
+    end
+  end
+
   def test_insert_logs_message_including_model_name
     skip unless supports_insert_conflict_target?
 
@@ -260,8 +288,18 @@ class InsertAllTest < ActiveRecord::TestCase
     assert_equal "New edition", Book.find(1).name
   end
 
-  def test_upsert_all_updates_existing_record_by_configured_primary_key
-    skip unless supports_insert_on_duplicate_update?
+  def test_upsert_all_does_notupdates_existing_record_by_when_there_is_no_key
+    skip unless supports_insert_on_duplicate_update? && !supports_insert_conflict_target?
+
+    Speedometer.create!(speedometer_id: "s3", name: "Very fast")
+
+    Speedometer.upsert_all [{ speedometer_id: "s3", name: "New Speedometer" }]
+
+    assert_equal "Very fast", Speedometer.find("s3").name
+  end
+
+  def test_upsert_all_updates_existing_record_by_configured_primary_key_fails_when_database_supports_insert_conflict_target
+    skip unless supports_insert_on_duplicate_update? && supports_insert_conflict_target?
 
     error = assert_raises ArgumentError do
       Speedometer.upsert_all [{ speedometer_id: "s1", name: "New Speedometer" }]

activerecord/test/models/cart.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+class Cart < ActiveRecord::Base
+  self.primary_key = :id
+end

activerecord/test/schema/schema.rb

@@ -152,6 +152,16 @@ ActiveRecord::Schema.define do
 
   create_table :carriers, force: true
 
+  create_table :carts, force: true, primary_key: [:shop_id, :id] do |t|
+    if current_adapter?(:Mysql2Adapter)
+      t.bigint :id, index: true, auto_increment: true, null: false
+    else
+      t.bigint :id, index: true, null: false
+    end
+    t.bigint :shop_id
+    t.string :title
+  end
+
   create_table :categories, force: true do |t|
     t.string :name, null: false
     t.string :type