When skipping duplicates in bulk insert on MySQL, avoid assigning id when not specified

If `id` is an `AUTONUMBER` column, then my former strategy here of assigning `no_op_column` to an arbitrary column would fail in this specific scenario:

1. `model.columns.first` is an AUTONUMBER column
2. `model.columns.first` is not assigned in the insert attributes

I added three tests: the first test covers the actual error; the second test documents that this _isn't_ a problem when a value is given for the AUTONUMBER column and the third test ensures that this no-op strategy isn't secretly doing an UPSERT.

1 files changed, 38 insertions, 0 deletions

diff --git a/activerecord/test/cases/insert_all_test.rb b/activerecord/test/cases/insert_all_test.rb
index fc25701c80..61fbda1f0f 100644
--- a/activerecord/test/cases/insert_all_test.rb
+++ b/activerecord/test/cases/insert_all_test.rb
@@ -104,6 +104,44 @@ class InsertAllTest < ActiveRecord::TestCase
     end
   end
 
+  def test_insert_all_with_skip_duplicates_and_autonumber_id_not_given
+    skip unless supports_insert_on_duplicate_skip?
+
+    assert_difference "Book.count", 1 do
+      # These two books are duplicates according to an index on %i[author_id name]
+      # but their IDs are not specified so they will be assigned different IDs
+      # by autonumber. We will get an exception from MySQL if we attempt to skip
+      # one of these records by assigning its ID.
+      Book.insert_all [
+        { author_id: 8, name: "Refactoring" },
+        { author_id: 8, name: "Refactoring" }
+      ]
+    end
+  end
+
+  def test_insert_all_with_skip_duplicates_and_autonumber_id_given
+    skip unless supports_insert_on_duplicate_skip?
+
+    assert_difference "Book.count", 1 do
+      Book.insert_all [
+        { id: 200, author_id: 8, name: "Refactoring" },
+        { id: 201, author_id: 8, name: "Refactoring" }
+      ]
+    end
+  end
+
+  def test_skip_duplicates_strategy_does_not_secretly_upsert
+    skip unless supports_insert_on_duplicate_skip?
+
+    book = Book.create!(author_id: 8, name: "Refactoring", format: "EXPECTED")
+
+    assert_no_difference "Book.count" do
+      Book.insert(author_id: 8, name: "Refactoring", format: "UNEXPECTED")
+    end
+
+    assert_equal "EXPECTED", book.reload.format
+  end
+
   def test_insert_all_will_raise_if_duplicates_are_skipped_only_for_a_certain_conflict_target
     skip unless supports_insert_on_duplicate_skip? && supports_insert_conflict_target?