Views with join on variable values

[ymarcon]

At the moment a view needs to be created for each entity type, the views exported and then combined afterwards. It would be very useful to have the ability to export views containing data of multiple entity types. This would require one of the tables in the view to have a Referenced Entity Type variable that creates the join between the two entities.

(from @sam-story, "Views with multiple entity types" and "Collapsing view data")

[ymarcon]

This would be like atomating and optimizing the use of the $join method. When creating a view, a table from another entity type could be added only if there is a variable with the corresponding "referenced entity type" which values would make the join. The selection of the ref variable must be explicit and if there is more than one ref variable for the given entity type, the one to be used must be selected by the view creator.

Then a derivation script such as:

$join('ds.table:var', 'refVar')

would be simplified to (after the join of "ds.table" on "refVar" has been declared):

$('var')

Note that the referred entity type is not necessarily different from the view's entity type. This is for the case where all entities are Participant type but we are interested in there relationship (mother, father, children, sibling, etc.).