cubicweb #1689727 Remove UI constraints from schema (?) [open]

As of today, to filter entities automatically in HTML selectors in edition forms, we can add constraints in the schema. This is really convenient, but sometimes unuseful from the data coherency point of view and may be costly in following example. Imagine EType has 2 relations with subject cardinality 1 named rel1 and rel2, which targets must be related together. We can add only one constraint on rel1 (in which rel2 object is present), but in this case rel2 select choices in UI will not be filtered enough. Adding the symmetrical constraint on rel2 solves the problem but implies code duplication and a performance penalty when saving the entity.

Still, it is much more convenient than writing a 'choices' form function and is not erroneous from the data coherency point of view... what should we do?

For a real example, see complexunits changeset 179216d75fad.

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