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, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index f5da19f0f6..ca8bbc14da 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -516,8 +516,8 @@ module ActiveRecord
         sql = +"INSERT #{insert.into} #{insert.values_list}"
 
         if insert.skip_duplicates?
-          any_column = quote_column_name(insert.model.columns.first.name)
-          sql << " ON DUPLICATE KEY UPDATE #{any_column}=#{any_column}"
+          no_op_column = quote_column_name(insert.keys.first)
+          sql << " ON DUPLICATE KEY UPDATE #{no_op_column}=#{no_op_column}"
         elsif insert.update_duplicates?
           sql << " ON DUPLICATE KEY UPDATE "
           sql << insert.updatable_columns.map { |column| "#{column}=VALUES(#{column})" }.join(",")