Support import/export in Recommendation module

[reckart]

Implement a ProjectExporter for the recommendation module:

• [x] recommender settings which are stored in the DB should be exported to/imported from the JSON structure

[jcklie]

@reckart Do you have an example importer/exporter that we can follow? I would put one into inception-recommondation. For the functionality, I would just go through the list of recommenders (inject the recommenderservice), turn them into json and save them via exportedProject.setProperty and vice versa.

[jcklie]

What happens when I persist a recommender which has a project field inside it? Do I need to set the project again from the outside? What happens to the database generated IDs of the imported stuff? What about the layer?

[reckart]

You could have a look e.g. at the LayerExporter and the TagSetExporter.

Exporters can have dependencies on other exporters (mind that exporters also import - don't worry about the name). So when your exporter depends e.g. on layers, then getImportDependencies needs to be overwritten and return a list containing the LayerExporter class.

Then during import, you need to look up already imported objects by their "business" key instead of their ID. E.g. layers have a unique name within the project, so you look them up by name. The target project to import to is passed as an argument to .ProjectExporter.importData(ProjectImportRequest, Project, ExportedProject, ZipFile).

[reckart]

Oh, and on export, you do not persist IDs, but you write the business key (e.g. the layer name). The project ID can be skipped because exports are per-project.

For every DB entity, we have a JSON DTO which is used during export/import (cf. LayerExporter and TagSetExporter).