Provide support for contextually-readonly properties, and odata services which don't follow Foreign Key BINDs

[jonnerd154]

These tweaks solve issues with support for SAP Business One's Service Layer. All edits welcome. I don't believe these changes have adverse impacts for other services, and are likely workarounds for problems in SAP B1. They solved my problems, but YMMV - more testing encouraged.

[eblis]

I can't test it with SAP as I don't have access to it.
I made some changes, added explanation about the new field and also made it not filter out everything when not in use.

Can you please test and see if it works for you?

[eblis]

Can you please check if the properties SAP fails for are marked as Nullable=false ?

[jonnerd154]

Your changes are great: type hints, documentation, improve code reuse, and the False case is smart. Thank you.

I pushed another commit that prevents calling .len() on a bool when omit_null_props=False.

Regarding the Nullable=False fields, unfortunately that's not the issue.

For example, the Orders entity has its CardCode property (the foreign key to BusinessPartner aka "Customer") spec'd as Nullable=False, and that validation works correctly: if it's null, the service returns an error.

The problem is that I get an error if I send property InterimType using it's initialized value as None/null. In the metadata spec, InterimType IS Nullable. To make matters more frustrating, if I get() the freshly inserted entity instance, InterimType is null; I'm not trying to change it on insert, and no change would have occurred, but the mere attempt at asserting its value, even if the value provided is the same as the default, triggers the error.

I think this could be best solved by improving the behavior of properties' __set__() and is_dirty() checking. If the client has attempted to set the value of a property, even if setting it to None, it should be included in the the request. If it's untouched, it is omitted from the request. This keeps the interface clean, and eliminates the need to manually list the offending properties or blindly omit all None values. Some people might actually want to set a value to Null explicitly.

Thoughts? I'd be happy to take a crack at it if you agree.

[eblis]

Sounds good. If you have the time go ahead.