facet_m2m gets confused by multiple relationships

[simonw]

I got this for a database I was playing with:

I think this is because of these three tables:

[simonw]

I think there are a few steps to solving this:

• Define a variant of ?_facet_m2m={..} that takes a JSON object which specifies which middle and destination table should be used, and via which columns
• Teach the Facet ManyToManyFacet.suggest() method to check for multiple conflicting m2m relationships and use the new JSON URL for them. Also pick non-conflicting display names for those facets.

[simonw]

Part of #551