Support NULL value replacement for custom types

zitnik commented 7 months ago

This PR aims to add support for NULL value replacement in custom types. Builds upon #39 and #44.

Current issue

The issue that I came across with the current implementation is that when the pagination cursor includes null for a custom type, that value is not properly replaced with the NullReplacement value. The reason is that the Go-representation of the cursor value is not nil, but a custom type object (with an underlying value of nil).

This in turn causes the pagination WHERE clause to compare the COALESCEd values of the target column with an unreplaced NULL, e.g. WHERE COALESCE(my_attr, '') > NULL.

Implementation

The change in this PR specifically handles custom types in the isNil function so that for custom types we evaluate isNil on the underlying value retrieved by GetCustomTypeValue instead of on the object itself.

Test

In order to construct a test, I added a custom type as an alias of sql.NullString, implemented the necessary methods on this type, and added a new field to the order struct. I guess this is a bit clumsy, in case you have any ideas about how to construct such a test more compactly, please let me know :) In the test itself, we have a primary pagination rule on "Remark" and a secondary rule on the new "Description" field so that the NULL element (ID 2) appears in the cursor causing the described problem.