Implement relationships & tweaks to improve Django admin

[EvdH0]

The pull request includes a proposal to add functionality to add or remove relationships between nodes. Many-to-Many relationships show up as a multiple select elements (and the horizontal/vertical filter widgets can be applied in the Django admin). Additionally minor improvements to the Django admin experience are included.

Features

• Add relationships as editable fields that work in the Django admin by using the Django App Registry
• Possibility of adding horizontal/vertical filter widgets to the relationships in the Django admin
• Delete button in the Django admin did not work, fixed
• Changed the pk property into an alias instead of a reference to the primary-key (because the Django internals do not address pk consistently in the same way). This allows general lookup (and for example fixes the delete button in Django admin)
• Date widget work now in the Django admin
• Expanded the test suite with few more cases

Limitations

• 'Multi select delete' actions in Django overview admin still doesn't work yet
• For DateFields a widget is shown in the admin, however for DateTime fields this widget is not shown but just a normal input box
• Cardinality constraints on relationships are not properly handled, currently works well for ZeroOrMore, but not for One. A relationship is currently also defined as a ManyToMany field, which is not correct if it would be a ZeroOrOne (would be a ForeignKey)
• Properties on relationships cannot be edited yet
• Choices work, but if an empty option is needed this needs to be defined in the choices of the constructor explicitly (ie STATUS = {'':'Empty','F': 'Female', 'M': 'Male', 'O': 'Other'}. Need to check if this is the intended behaviour in neomodel

[werneckpaiva]

Any plans to accept this pull request?

[mariusconjeaud]

I am looking at it now @EvdH0 @werneckpaiva. As they say, better late than never. Even when very mega insanely late ?

[mariusconjeaud]

Tests fail for Python 3.10 and above with : AttributeError: 'NeoNodeSet' object has no attribute '_prefetch_related_lookups'

Looks like it is because the currently used version of Django is too old. I'll see if I can upgrade.

Fixed things ; the problem was actually that the modifications introduced indirect calls to the prefetch mechanism, which was not implemented in django-neomodel.
Added a "passthrough" implementation, just to prevent it from failing. It would be interesting to actually implement it though.

[mariusconjeaud]

Tests now pass ; but if you open the admin interface and try to list / add some Book / Shelf / Author (so the classes that are modified by this), then it fails.