WIP: Support Filtering using ne:null on nullable attribute

[NullVoxPopuli]

BUG FIX

• reproduce issue in tests
• fix issue
• bump package version

The root of this was that I wanted to be able to ?filter[nullable-field]=ne:, meaning that I wanted all resources where nullable-field had a value.

I had originally tried using ?filter[nullable-field]=ne:null but, this was interpreting null as "null", which resulted in the error: Could not cast null to Nullable'1.

Since checking if something is not null can be handy, I added a check for the Nullable generic type, and if the value is an empty string, swap it out for null. :-\

This may be a breaking change. idk.

[NullVoxPopuli]

what if I add two FilterOperations for notnull and @null?

[milosloub]

@NullVoxPopuli good idea! But this issue with null is quite confusing.
In case of ?filter[nullable-attr]=notnull (or null) and its FilterOperation (notnull or @null)
leads to Resource.NullableAttr != "notnull". It's because of default eq operator, when there is no
FilterOperation provided. So to fix this, you should have route like this:
?filter[nullable-attr]=notnull:someIrrelevantValue.

I think you were right with previous solution ?filter[nullable-attr]=NULL or filter[nullable-attr]=ne:NULL.

Edit: For error Could not cast null to Nullable'1,
instead of this:
var value = isNullabe && propertyValue == "" ? null : propertyValue;
try:
var value = isNullabe && propertyValue == "" ? (property.PropertyType)null : propertyValue;

Ternary operator should return same type of value.

[NullVoxPopuli]

I think you were right with previous solution ?filter[nullable-attr]=NULL or filter[nullable-attr]=ne:NULL

but what if someone wants to test for the string value of "NULL"

[NullVoxPopuli]

Maybe something like this?

(just now value on the right side of the colon)

[milosloub]

Yeah, thats the problem of string stored NULL values.
What about something like filter[attr]=is:null and filter[attr]=isnot:null?
This is similar to SQL queries

[NullVoxPopuli]

what's to stop the usage of is:somethingelse? :-\ would we hard-code checks on null value usage?

[milosloub]

Here is null value like every other value after colon. So there must be conversion to null in this case. Maybe someone comes with better idea.
In case of is:somethingElse...its should be the same as eq:somethingElse. I didnt try the difference between Items.Where(i => i.Attr == value) and Items.Where(i => i.Attr.Equals(value)) from EF Core point of view.
I dont know if EF core translate Items.Where(i => i.Attr.Equals(null)) to SQL: "WHERE Attr IS NULL" by default.
If yes, operator "is" or "isnot" can be used to build Expression based od Equals Method.

[NullVoxPopuli]

cool. I'll submit a different PR with is and isnot