Warn if `AR.primary_key` is called for a table who has composite primary key

If `AR.primary_key` is called for a table who has composite primary key,
the method returns `nil`. This behavior sometimes generates invalid SQL.
The first time developers notice to invalid SQL is when they execute
SQL. This commit enables developers to know they are doing something
dangerous as soon as possible.

2 files changed, 13 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index c7aff63228..3b4ceb915e 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -122,6 +122,12 @@ module ActiveRecord
       # Returns just a table's primary key
       def primary_key(table_name)
         pks = primary_keys(table_name)
+        warn <<-WARNING.strip_heredoc if pks.count > 1
+          WARNING: Rails does not support composite primary key.
+
+          #{table_name} has composite primary key. Composite primary key is ignored.
+        WARNING
+
         pks.first if pks.one?
       end
 
diff --git a/activerecord/test/cases/primary_keys_test.rb b/activerecord/test/cases/primary_keys_test.rb
index 7e18313c00..360efd0b2b 100644
--- a/activerecord/test/cases/primary_keys_test.rb
+++ b/activerecord/test/cases/primary_keys_test.rb
@@ -262,6 +262,13 @@ class CompositePrimaryKeyTest < ActiveRecord::TestCase
     assert_equal ["region", "code"], @connection.primary_keys("barcodes")
   end
 
+  def test_primary_key_issues_warning
+    warning = capture(:stderr) do
+      @connection.primary_key("barcodes")
+    end
+    assert_match(/WARNING: Rails does not support composite primary key\./, warning)
+  end
+
   def test_collectly_dump_composite_primary_key
     schema = dump_table_schema "barcodes"
     assert_match %r{create_table "barcodes", primary_key: \["region", "code"\]}, schema