title: mongodb example: why is this not a covered query?
body: <p>i created the following collection:</p>
  <pre><code>students&gt; db.students.find()
  [
    { _id: 1, grades: [ 95, 92, 90 ], average: 145 },
    { _id: 2, grades: [ 98, 100, 102 ], average: 145 },
    { _id: 3, grades: [ 95, 110, 100 ], average: 145 }
  ]
  </code></pre>
  <p>these are the available indexes:</p>
  <pre><code>students&gt; db.students.getIndexes()
  [
    { v: 2, key: { _id: 1 }, name: '_id_' },
    { v: 2, key: { grades: 1 }, name: 'grades_1' },
    { v: 2, key: { average: 1 }, name: 'average_1' }
  ]
  </code></pre>
  <p>If i run this query, i expect it to be covered, since it needs just the 'average' field, which is an indexed field.</p>
  <pre><code>db.students.find({average:145}, {_id:0, grades:0})
  </code></pre>
  <p>Instead, i get a the following output using explain(&quot;executionStats&quot;):</p>
  <pre><code>[...]
  winningPlan: {
        stage: 'PROJECTION_DEFAULT',
        transformBy: { _id: 0, grades: 0 },
        inputStage: {
          stage: 'FETCH',
          inputStage: {
            stage: 'IXSCAN',
            keyPattern: { average: 1 },
            indexName: 'average_1',
            isMultiKey: false,
            multiKeyPaths: { average: [] },
            isUnique: false,
            isSparse: false,
            isPartial: false,
            indexVersion: 2,
            direction: 'forward',
            indexBounds: { average: [ '[145, 145]' ] }
          }
        }
      }
  </code></pre>
  <p>And this:</p>
  <pre><code> totalDocsExamined: 3
  </code></pre>
  <p>Which means that the query had to read the documents in the collection... what am i missing?
  Thanks</p>