`connection.type_to_sql` returns a `String` for unmapped types.

Closes #13146.

This fixes an error when using:

```
change_colum :table, :column, :bigint, array: true
```

3 files changed, 16 insertions, 1 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 59d409d04d..936dcbb2fb 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,14 @@
+*   `type_to_sql` returns a `String` for unmapped columns. This fixes an error
+    when using unmapped array types in PG
+
+    Example:
+
+        change_colum :table, :column, :bigint, array: true
+
+    Fixes #13146.
+
+    *Jens Fahnenbruck*, *Yves Senn*
+
 *   Fix `QueryCache` to work with nested blocks, so that it will only clear the existing cache
     after leaving the outer block instead of clearing it right after the inner block is finished.
 
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 6268ae4875..51a6929dab 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -690,7 +690,7 @@ module ActiveRecord
 
           column_type_sql
         else
-          type
+          type.to_s
         end
       end
 
diff --git a/activerecord/test/cases/adapter_test.rb b/activerecord/test/cases/adapter_test.rb
index 595edc6263..34be68a97f 100644
--- a/activerecord/test/cases/adapter_test.rb
+++ b/activerecord/test/cases/adapter_test.rb
@@ -178,6 +178,10 @@ module ActiveRecord
       result = @connection.select_all "SELECT * FROM posts"
       assert result.is_a?(ActiveRecord::Result)
     end
+
+    test "type_to_sql returns a String for unmapped types" do
+      assert_equal "special_db_type", @connection.type_to_sql(:special_db_type)
+    end
   end
 
   class AdapterTestWithoutTransaction < ActiveRecord::TestCase