MyXQL: Use INSERT IGNORE for on_conflict: :nothing

The previous implementation used `ON DUPLICATE KEY UPDATE col = col` which
had incorrect semantics:

1. It reported 1 row affected even when a row was skipped due to a duplicate
   key conflict, because the UPDATE clause still matched the existing row
2. This caused insert_all to return {1, nil} instead of {0, nil} for ignored
   duplicates, misrepresenting the actual number of inserted records
3. The UPDATE clause could trigger unnecessary row-level operations

This change:

- Uses `INSERT IGNORE INTO` which properly ignores duplicate key conflicts
  without affecting existing rows or incrementing the affected row count
- Handles num_rows: 0 in the adapter by returning {:ok, []} for
  on_conflict: :nothing, since 0 rows is expected behavior when all rows
  are duplicates
- Updates tests to verify correct row counts: {0, nil} for all-duplicates,
  {N, nil} for N successfully inserted non-duplicate rows

Author: mehiel

integration_test/myxql/upsert_all_test.exs

@@ -13,10 +13,63 @@ defmodule Ecto.Integration.UpsertAllTest do
 
   test "on conflict ignore" do
     post = [title: "first", uuid: "6fa459ea-ee8a-3ca4-894e-db77e160355e"]
-    assert TestRepo.insert_all(Post, [post], on_conflict: :nothing) ==
-           {1, nil}
-    assert TestRepo.insert_all(Post, [post], on_conflict: :nothing) ==
-           {1, nil}
+    # First insert succeeds - 1 row inserted
+    assert TestRepo.insert_all(Post, [post], on_conflict: :nothing) == {1, nil}
+    # Second insert is ignored due to duplicate - 0 rows inserted (INSERT IGNORE behavior)
+    assert TestRepo.insert_all(Post, [post], on_conflict: :nothing) == {0, nil}
+  end
+
+  test "on conflict ignore with mixed records (some conflicts, some new)" do
+    # Insert an existing post
+    existing_uuid = "6fa459ea-ee8a-3ca4-894e-db77e160355e"
+    existing_post = [title: "existing", uuid: existing_uuid]
+    assert TestRepo.insert_all(Post, [existing_post], on_conflict: :nothing) == {1, nil}
+
+    # Now insert a batch with one duplicate and two new records
+    new_uuid1 = "7fa459ea-ee8a-3ca4-894e-db77e160355f"
+    new_uuid2 = "8fa459ea-ee8a-3ca4-894e-db77e160355a"
+
+    posts = [
+      [title: "new post 1", uuid: new_uuid1],     # new - should be inserted
+      [title: "duplicate", uuid: existing_uuid],   # duplicate - should be ignored
+      [title: "new post 2", uuid: new_uuid2]       # new - should be inserted
+    ]
+
+    # With INSERT IGNORE, only 2 rows should be inserted (the non-duplicates)
+    assert TestRepo.insert_all(Post, posts, on_conflict: :nothing) == {2, nil}
+
+    # Verify the data - should have 3 posts total (1 existing + 2 new)
+    assert length(TestRepo.all(Post)) == 3
+
+    # Verify the existing post was not modified
+    [original] = TestRepo.all(from p in Post, where: p.uuid == ^existing_uuid)
+    assert original.title == "existing"  # title unchanged
+
+    # Verify new posts were inserted
+    assert TestRepo.exists?(from p in Post, where: p.uuid == ^new_uuid1)
+    assert TestRepo.exists?(from p in Post, where: p.uuid == ^new_uuid2)
+  end
+
+  test "on conflict ignore with all duplicates" do
+    # Insert initial posts
+    uuid1 = "1fa459ea-ee8a-3ca4-894e-db77e160355e"
+    uuid2 = "2fa459ea-ee8a-3ca4-894e-db77e160355e"
+    initial_posts = [
+      [title: "first", uuid: uuid1],
+      [title: "second", uuid: uuid2]
+    ]
+    assert TestRepo.insert_all(Post, initial_posts, on_conflict: :nothing) == {2, nil}
+
+    # Try to insert all duplicates
+    duplicate_posts = [
+      [title: "dup1", uuid: uuid1],
+      [title: "dup2", uuid: uuid2]
+    ]
+    # All are duplicates, so 0 rows inserted
+    assert TestRepo.insert_all(Post, duplicate_posts, on_conflict: :nothing) == {0, nil}
+
+    # Verify count unchanged
+    assert length(TestRepo.all(Post)) == 2
   end
 
   test "on conflict keyword list" do

lib/ecto/adapters/myxql.ex

@@ -330,9 +330,17 @@ defmodule Ecto.Adapters.MyXQL do
 
     case Ecto.Adapters.SQL.query(adapter_meta, sql, values ++ query_params, opts) do
       {:ok, %{num_rows: 0}} ->
-        raise "insert operation failed to insert any row in the database. " <>
-                "This may happen if you have trigger or other database conditions rejecting operations. " <>
-                "The emitted SQL was: #{sql}"
+        # With INSERT IGNORE (on_conflict: :nothing), 0 rows means the row was
+        # ignored due to a conflict, which is expected behavior
+        case on_conflict do
+          {:nothing, _, _} ->
+            {:ok, []}
+
+          _ ->
+            raise "insert operation failed to insert any row in the database. " <>
+                    "This may happen if you have trigger or other database conditions rejecting operations. " <>
+                    "The emitted SQL was: #{sql}"
+        end
 
       # We were used to check if num_rows was 1 or 2 (in case of upserts)
       # but MariaDB supports tables with System Versioning, and in those

lib/ecto/adapters/myxql/connection.ex

@@ -181,9 +181,10 @@ if Code.ensure_loaded?(MyXQL) do
     @impl true
     def insert(prefix, table, header, rows, on_conflict, [], []) do
       fields = quote_names(header)
+      insert_keyword = insert_keyword(on_conflict)
 
       [
-        "INSERT INTO ",
+        insert_keyword,
         quote_table(prefix, table),
         " (",
         fields,
@@ -192,6 +193,9 @@ if Code.ensure_loaded?(MyXQL) do
       ]
     end
 
+    defp insert_keyword({:nothing, _, []}), do: "INSERT IGNORE INTO "
+    defp insert_keyword(_), do: "INSERT INTO "
+
     def insert(_prefix, _table, _header, _rows, _on_conflict, _returning, []) do
       error!(nil, ":returning is not supported in insert/insert_all by MySQL")
     end
@@ -208,9 +212,9 @@ if Code.ensure_loaded?(MyXQL) do
       []
     end
 
-    defp on_conflict({:nothing, _, []}, [field | _]) do
-      quoted = quote_name(field)
-      [" ON DUPLICATE KEY UPDATE ", quoted, " = " | quoted]
+    defp on_conflict({:nothing, _, []}, _header) do
+      # Handled by INSERT IGNORE
+      []
     end
 
     defp on_conflict({fields, _, []}, _header) when is_list(fields) do

test/ecto/adapters/myxql_test.exs

@@ -1465,11 +1465,10 @@ defmodule Ecto.Adapters.MyXQLTest do
     end
   end
 
-  test "insert with on duplicate key" do
+  test "insert with on conflict" do
+    # Using INSERT IGNORE for :nothing on_conflict
     query = insert(nil, "schema", [:x, :y], [[:x, :y]], {:nothing, [], []}, [])
-
-    assert query ==
-             ~s{INSERT INTO `schema` (`x`,`y`) VALUES (?,?) ON DUPLICATE KEY UPDATE `x` = `x`}
+    assert query == ~s{INSERT IGNORE INTO `schema` (`x`,`y`) VALUES (?,?)}
 
     update = from("schema", update: [set: [z: "foo"]]) |> plan(:update_all)
     query = insert(nil, "schema", [:x, :y], [[:x, :y]], {update, [], []}, [])