Performing a "Select where property equals A or B" in CouchDB

I've been slowly converting a few SQL database tables to CouchDB where it makes sense, and recently ran into a problem where I couldn't figure out how to translate a simple SQL select statement into the CouchDB equivalant, where I wanted to select documents that had one of two known values. It turns out this was pretty darn simple, but first let's look at what I was trying to query.

Pretend I've got a CouchDb database with these four documents:

[
    { "_id": "foo",
      "sourceId": 5,
      "targetId": 10 },
      
    { "_id": "bar",
      "sourceId": null,
      "targetId": 10 },

    { "_id": "baz",
      "sourceId": 3,
      "targetId": 10 },

    { "_id": "bat",
      "sourceId": null,
      "targetId": 15 }
]

I was trying to write a view function that would be the equivalant of the SQL Select * from table where targetId = 10 AND (where sourceId = 5 OR where sourceId = null). I wanted to select the documents with id foo or bar, but not the documents with id baz or bat.

To do that, I tried to write a view function that would emit the source id and target id as keys for each document:

function (doc) {
    emit([doc.targetId, doc.sourceId], doc)
}

That function would put out keys like [10, 5] and [10, null]. But I couldn't figure out how to actually make use of those keys to get the foo and bar documents without making two different requests. I had attempted to use wildcards in the query, but that was accidentally catching any documents that had source or target ids between the wildcards.

http://localhost:5984/database/_design/docname/view/_viewname?start_key=[10, 15]&end_key=[10, {}]

But that was accidentally picking up documents with any target id, not just one that was null or 15. It turns out I was massively overthinking this, because I somehow forgot that you don't have to use start keys and end keys; you can just specify the exact keys you're searching for!

So if I'm looking for documents with a target id of 10 and a source id that was either 5 or null, I can easily request just those using my view function and the keys=[...] parameter:

http://localhost:5984/database/_design/docname/view/_viewname?keys=[[10, 5], [10, null]]

That's all it takes to query my view function and get just the foo and bar documents!