MongoDB $lookup not using index : <p>I'm writing a query that requires a $lookup between two tables and as I understand it, it's essential that the foreignField have an index in order to perform this join in a timely fashion. However, even after adding an index on the field, the query is still falling back to COLLSCAN.</p>

<pre><code>db.users.aggregate([
  {$lookup:{ from: "transactions", localField: '_id', foreignField: 'uid', as: 'transaction' }},
  { $match: { transaction: { "$size" : 0} } },
  { $count: "total"},
], { explain: true })
</code></pre>

<p>This returns:</p>

<pre><code>"queryPlanner" : {
    "plannerVersion" : 1,
    "namespace" : "test.users",
    "indexFilterSet" : false,
    "parsedQuery" : {

    },
    "winningPlan" : {
        "stage" : "COLLSCAN",
        "direction" : "forward"
    },
    "rejectedPlans" : [ ]
}
</code></pre>

<p>As I mentioned, I do have the <em>uid</em> field indexed in the transactions collection:</p>

<pre><code>&gt; db.transactions.getIndexes()
[
    {
        "v" : 1,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "test.transactions"
    },
    {
        "v" : 1,
        "key" : {
            "uid" : 1
        },
        "name" : "uid_1",
        "ns" : "test.transactions"
    }
]
</code></pre>

<p>The query takes a few minutes to run in a DB of approximately 7M documents. I'm using MongoDB v3.4.7. Any idea as to what I could be doing wrong? Thanks in advance!</p>
