Fix has_been_used? for Postgres table names near the identifier length limit

lib/database_cleaner/active_record/truncation.rb

@@ -226,20 +226,27 @@ def database_cleaner_table_cache
         # but then the table is cleaned.  In other words, this function tells us if the given table
         # was ever inserted into.
         def has_been_used?(table)
-          return has_rows?(table) unless has_sequence?(table)
+          sequence = serial_sequence(table)
+          return has_rows?(table) unless sequence
 
-          cur_val = select_value("SELECT last_value from #{table}_id_seq;").to_i
+          cur_val = select_value("SELECT last_value from #{sequence};").to_i
           cur_val > 0
         end
 
-        def has_sequence?(table)
-          select_value("SELECT true FROM pg_class WHERE relname = '#{table}_id_seq';")
-        end
-
         def has_rows?(table)
           select_value("SELECT true FROM #{table} LIMIT 1;")
         end
 
+        # Looks up the real sequence name instead of guessing "#{table}_id_seq", which can be
+        # wrong once Postgres truncates it to fit its 63-byte identifier limit.
+        def serial_sequence(table)
+          select_value("SELECT pg_get_serial_sequence(#{quote(table)}, 'id')")
+        rescue ::ActiveRecord::StatementInvalid => e
+          raise unless e.cause.is_a?(PG::UndefinedColumn) # no "id" column at all
+
+          nil
+        end
+
         def tables_with_schema
           rows = select_rows <<-_SQL
             SELECT schemaname || '.' || tablename

spec/database_cleaner/active_record/truncation_spec.rb

@@ -98,6 +98,36 @@
             expect(strategy.send(:connection)).to receive(:truncate_tables).with(['users'], {:truncate_option=>:restrict})
             strategy.clean
           end
+
+          if helper.db == :postgres
+            context "with a table name at Postgres' identifier length limit" do
+              # :only this table: the rest of this file leaves "users"/"user_profiles" in whatever
+              # state earlier (mocked) examples left them in, and "user_profiles" has a real FK
+              # onto "users" that RESTRICT would trip on unrelated to what's under test here.
+              subject(:strategy) { described_class.new(pre_count: true, only: [long_table_name]) }
+              let(:long_table_name) { "u" * 58 + "_long" } # 63 chars: Postgres' NAMEDATALEN limit
+
+              before do
+                connection.execute("CREATE TABLE #{long_table_name} (id SERIAL PRIMARY KEY, name INTEGER);")
+              end
+
+              after do
+                connection.execute("DROP TABLE IF EXISTS #{long_table_name};")
+              end
+
+              it "does not raise when checking whether the table has been used" do
+                expect { strategy.clean }.not_to raise_error
+              end
+
+              it "detects activity on the table via its actual (differently-truncated) sequence" do
+                connection.execute("INSERT INTO #{long_table_name} (name) VALUES (1);")
+                connection.execute("INSERT INTO #{long_table_name} (name) VALUES (2);")
+
+                expect(strategy.send(:connection)).to receive(:truncate_tables).with([long_table_name], {truncate_option: :restrict})
+                strategy.clean
+              end
+            end
+          end
         end
 
         describe "with truncate_option set to cascade" do