Automatically generate IDs and UIDs for entities and properties

[nalenz-objectbox]

• when writing an entity class, it should be enough to specify the respective annotation without any parameters, i.e. @Entity() class MyEntity { ... } should suffice
• all instance variables of the entity class shall be incorporated into the final model and only those with special properties (e.g. @Id()) need to be annotated
• all IDs and UIDs need to be generated by objectbox_model_generator, together with these rules on how to resolve changes and conflicts
• in contrast to the current situation, this means that the existing generated file (specifically the _MyEntity_OBXModel object) needs to be parsed beforehand in order to detect changes

[vaind]

Have a look at https://github.com/objectbox/objectbox-go/tree/master/internal/generator as a reference implementation.

• binding.go analyzes source code and prepares binding structs
• package modelinfo handles reading and writing objectbox-model.json files (not in scope of this issue but will be necessary in the future)
• merge.go takes model information from the binding.go and updates modelinfo, also updating binding information, especially ID/UIDs

[nalenz-objectbox]

The branch 18-automatic-id-generation now contains the following changes:

• a file called objectbox-models.json is generated with a structure similar to ObjectBox Java
  • only merging entity properties is supported right now, indexes and relations are still completely missing
  • the overall file structure of the new generator is heavily influenced by the Go source you provided the link to
• unannotated fields in an @Entity() class now behave just like those annotated with @Property()
• upgraded Flutter demo with these new features

[vaind]

Oh, it's huge already. Please create a pull request (18-... to dev) so that I can review properly. And restrain yourself :) from adding more stuff (if you planned to do so) unless necessary for this issue - it's going to be hard to review as it is now. Thanks

[Buggaboo]

At the risk of increasing the lines of code here, are there plans to support other types other than String and int? Currently my query tests are failing because things like bool and double are not supported.

[vaind]

At the risk of increasing the lines of code here, are there plans to support other types other than String and int? Currently my query tests are failing because things like bool and double are not supported.

It's not really related to the original goal of this ticket and should rather be covered by #11 - so that we can merge this PR ASAP.