DOCS-7854: Document how to check if a field itself is an array

source/reference/operator/query/type.txt

@@ -320,17 +320,74 @@ The ``SensorReading`` collection contains the following documents:
          24,
          23
       ]
+   },
+   {
+      "_id": 3,
+      "readings": [
+         22,
+         24,
+         []
+      ]
+   },
+   {
+      "_id": 4,
+      "readings": []
+   },
+   {
+      "_id": 5,
+      "readings": 24
    }
-
-The following query returns any document where ``readings`` has an
-element of :term:`BSON` type ``array``; i.e. the :query:`$type` does
-not check if ``readings`` itself is an array:
+
+The following query returns any document in which the ``readings``
+field contains an element of :term:`BSON` type ``array``. Note that
+the :query:`$type` query filter does not detect whether the ``readings``
+field itself is an array.
 
 .. code-block:: javascript
 
    db.SensorReading.find( { "readings" : { $type: "array" } } )
 
-This returns the following document:
+The above query returns the following documents:
+
+.. code-block:: javascript
+
+   {
+      "_id": 1,
+      "readings": [
+         25,
+         23,
+         [ "Warn: High Temp!", 55 ],
+         [ "ERROR: SYSTEM SHUTDOWN!", 66 ]
+      ]
+   },
+   {
+      "_id": 3,
+      "readings": [
+         22,
+         24,
+         []
+      ]
+   }
+
+In the documents with ``_id : 1`` and ``_id : 3``, the ``readings``
+field contains at least one element which is an array, while the
+other documents do not.
+
+As seen above, the :query:`$type` operator looks inside a given field
+to see whether any of its elements are arrays, but it does not consider
+whether the given field is itself an array. To do that, use the
+following query:
+
+.. code-block:: javascript
+
+   db.SensorReading.find( { "readings" : { $elemMatch: { $exists: true } } } )
+
+:query:`$elemMatch` searches an array for a given element, and
+:query:`$exists` determines whether a given field exists. Used
+together, they return ``true`` if the specified field is of
+:term:`BSON` type ``array``.
+
+The above query returns the following documents:
 
 .. code-block:: javascript
 
@@ -342,10 +399,41 @@ This returns the following document:
          [ "Warn: High Temp!", 55 ],
          [ "ERROR: SYSTEM SHUTDOWN!", 66 ]
       ]
+   },
+   {
+      "_id": 2,
+      "readings": [
+         25,
+         25,
+         24,
+         23
+      ]
+   },
+   {
+      "_id": 3,
+      "readings": [
+         22,
+         24,
+         []
+      ]
    }
 
-The document with ``_id : 1`` has at least one element in ``readings``
-that is an ``array``, whereas the document with ``_id : 2`` does *not*.
+To find all documents in the collection in which the ``readings``
+field is an array, either empty or non-empty, the following query
+uses an :query:`$or` operator:
+
+.. code-block:: javascript
+
+   db.SensorReading.find( {
+     $or: [
+       { "readings": { $elemMatch: { $exists: true } } },
+       { "readings": [] }
+     ]
+   } )
+
+The above query returns the first four documents in the collection and
+excludes the one with ``_id : 5``, in which the ``readings`` field is
+an integer instead of an array.
 
 Additional Information
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