Allow to display tabular data by combining with geometries

[yohanboniface]

The very common use case we have is: someone works on a spreadsheet, so without geometries, but including some geographical reference (eg. in France an INSEE code).

We'd like to be able to display this data in uMap by letting the user combine their data to geometries with a common reference column.

This needs a bit of thinking.

A few options I can think of:

• first implement layer grouping, and then in one group to define a "join column", so all "sub layers" are merged/combined. This option is quite generic, and could allow other usages (like combining OSM data with local data, so for exemple to add subjective data to it, like stars or "my opinion about this restaurant")
• add a geometryUrl field in each layer (but geometries must be only URL, not local; that may be a feature, so to prevent copy-pasting again and again the same geometries)
• following that last option, we could have a drop down list of known geometries (that could still be stored locally), so one could choose one from the list (eg. "French communes") and a "union key" (we may want to prevent loading all the geometries dataset also)
• one could have a "depends on/union with" field on a layer, which would point to another layer we should combine (are there cases where we would want to combine more than two layers ?)
• today, layers are either saved locally or remote, we could allow to have both, and in this case merge features by some key

A few questions:

• how do we deal with non symmetric datasets (one have more data than the other) ?
• could we prevent loading all French communes for a dataset that needs only a few dozens of them ?
• are there scenarios where we want to union more than two layers ?