DOCS-368 Elemmatch and positional projection operators

source/reference/operator/positional.txt

@@ -4,24 +4,36 @@
 
 .. default-domain:: mongodb
 
+This page documents the use of the ``positional $`` operator to
+identify the array element to update in the :method:`update()
+<db.collection.update()>` method. For the positional ``$`` projection
+operator used to return an array element from a read operation, see the
+:doc:`positional $ </reference/projection/positional>` projection
+operator.
+
 .. operator:: $
 
    *Syntax*: ``{ "<array>.$" : value }``
 
    The positional :operator:`$` operator identifies an element in an
    ``array`` field to update without explicitly specifying the position
-   of the element in the array. The positional :operator:`$` operator,
-   when used with the :method:`update()
-   <db.collection.update()>` method and acts as a placeholder for the
-   **first match** of the update ``query selector``:
+   of the element in the array. 
 
-   .. code-block:: javascript
+   When used with the :method:`update() <db.collection.update()>`
+   method,
+
+   - the positional :operator:`$` operator acts as a placeholder for
+     the **first** element that matches the :ref:`query document
+     <mongodb-query-document>`, and
 
-      db.collection.update( { <query selector> }, { <update operator>: { "array.$" : value } } )
+   - the ``array`` field **must** appear as part of the ``query
+     document``.
+
+   .. code-block:: javascript
 
-   The ``array`` field **must** appear as part of the ``query selector``.
+      db.collection.update( { <array>: value ... }, { <update operator>: { "<array>.$" : value } } )
 
-   Consider the following collection ``students`` with the following documents:
+   Consider a collection ``students`` with the following documents:
 
    .. code-block:: javascript
 
@@ -38,7 +50,8 @@
       db.students.update( { _id: 1, grades: 80 }, { $set: { "grades.$" : 82 } } )
 
    Remember that the positional :operator:`$` operator acts as a
-   placeholder for the **first match** of the update ``query selector``.
+   placeholder for the **first match** of the update :ref:`query
+   document <mongodb-query-document>`.
 
    The positional :operator:`$` operator facilitates updates to arrays
    that contain embedded documents. Use the positional :operator:`$`
@@ -66,18 +79,17 @@
 
       db.students.update( { _id: 4, "grades.grade": 85 }, { $set: { "grades.$.std" : 6 } } )
 
-   Consider the following behaviors when using the positional
-   :operator:`$` operator:
-
-   - Do not use the positional operator :operator:`$` with
-     :term:`upsert` operations because, inserts will use the ``$`` as a field name
-     in the inserted document.
+   .. note::
+
+      - Do not use the positional operator :operator:`$` with
+        :term:`upsert` operations because inserts will use the ``$`` as
+        a field name in the inserted document.
 
-   - When used with the :operator:`$unset` operator, the positional
-     :operator:`$` operator does not remove the matching element from
-     the array but rather sets it to ``null``.
+      - When used with the :operator:`$unset` operator, the positional
+        :operator:`$` operator does not remove the matching element
+        from the array but rather sets it to ``null``.
 
    .. seealso::
-   
+
       :method:`update() <db.collection.update()>`, :operator:`$set` and
       :operator:`$unset`

source/reference/projection/elemMatch.txt

@@ -10,49 +10,158 @@ $elemMatch (projection)
 
    .. versionadded:: 2.2
 
-   Use the :projection:`$elemMatch` projection operator to limit the
-   response of a query to a single matching element of an
-   array. Consider the following:
+   The :projection:`$elemMatch` projection operator limits the contents
+   of an array field that is included in the query results to contain
+   only the element that matches the query condition.
+
+   *Syntax*: ``{ $elemMatch: { <array>: <value> } }``
+
+   The ``<value>`` can be documents that contains :ref:`query operator
+   expressions <query-selectors-comparison>`.
+
+   .. note::
+
+      - If multiple elements match the :projection:`$elemMatch`
+        condition, the operator returns the **first** matching element
+        in the ``<array>`` field.
+
+      - The :projection:`$elemMatch` projection operator is similar to the
+        :doc:`positional $ </reference/projection/positional>`
+        projection operator.
+
+   The examples on the :projection:`$elemMatch` projection operator
+   assumes a collection ``school`` with the following documents:
+
+   .. code-block:: javascript
+
+      {
+       _id: 1,
+       zipcode: 63109,
+       students: [
+                    { name: "john", school: 102, age: 10 },
+                    { name: "jess", school: 102, age: 11 },
+                    { name: "jeff", school: 108, age: 15 }
+                 ]
+      }
+      {
+       _id: 2,
+       zipcode: 63110,
+       students: [
+                    { name: "ajax", school: 100, age: 7 },
+                    { name: "achilles", school: 100, age: 8 },
+                 ]
+      }
+
+      {
+       _id: 3,
+       zipcode: 63109,
+       students: [
+                    { name: "ajax", school: 100, age: 7 },
+                    { name: "achilles", school: 100, age: 8 },
+                 ]
+      }
+
+      {
+       _id: 4,
+       zipcode: 63109,
+       students: [
+                    { name: "barney", school: 102, age: 7 },
+                 ]
+      }
 
    .. example::
 
-      Given the following document fragment:
+      The following :method:`find() <db.collection.find()>` operation
+      queries for all documents where the value of the ``zipcode``
+      field is ``63109``. The :projection:`$elemMatch` projection
+      returns only the **first** matching element of the ``students``
+      array where the ``school`` field has a value of ``102``:
 
       .. code-block:: javascript
 
-         {
-          _id: ObjectId(),
-          zipcode: 63109,
-          dependents: [
-                       { name: "john", school: 102, age: 10 },
-                       { name: "jess", school: 102, age: 11 },
-                       { name: "jeff", school: 108, age: 15 }
-                      ]
-         }
+         db.schools.find( { zipcode: 63109 }, 
+                          { students: { $elemMatch: { school: 102 } } } )
 
-      Consider the following :method:`find() <db.collection.find()>`
-      operation:
+      The operation returns the following documents:
 
       .. code-block:: javascript
 
-         var projection = { _id: 0, dependents: { $elemMatch: { school: 102 }}};
-         db.students.find( { zipcode: 63109 }, projection);
+         { "_id" : 1, "students" : [ { "name" : "john", "school" : 102, "age" : 10 } ] }
+         { "_id" : 3 }
+         { "_id" : 4, "students" : [ { "name" : "barney", "school" : 102, "age" : 7 } ] }
 
-      The query would return all documents where the value of the
-      ``zipcode`` field is ``63109``, while the projection excludes
-      the ``_id`` field and only includes the first matching element of
-      the ``dependents`` array where the ``school`` element has a value of
-      ``102``. The documents would take the following form:
+      - For the document with ``_id`` equal to ``1``, the ``students``
+        array contains multiple elements with the ``school`` field
+        equal to ``102``. However, the :projection:`$elemMatch`
+        projection returns only the first matching element from the
+        array.
+
+      - The document with ``_id`` equal to ``3`` does not contain the
+        ``students`` field in the result since no element in its
+        ``students`` array matched the :projection:`$elemMatch`
+        criterion.
+
+   The :projection:`$elemMatch` projection can specify criteria on multiple
+   fields:
+
+   .. example::
+
+      The following :method:`find() <db.collection.find()>` operation
+      queries for all documents where the value of the ``zipcode``
+      field is ``63109``. The projection includes the **first**
+      matching element of the ``students`` array where the ``school``
+      field has a value of ``102`` **and** the ``age`` field is greater
+      than ``10``:
+
+      .. code-block:: javascript
+
+         db.schools.find( { zipcode: 63109 }, 
+                          { students: { $elemMatch: { school: 102, age: { $gt: 10} } } } )
+
+      The operation returns the three documents that have ``zipcode`` equal to ``63109``:
+
+      .. code-block:: javascript
+
+         { "_id" : 1, "students" : [ { "name" : "jess", "school" : 102, "age" : 11 } ] }
+         { "_id" : 3 }
+         { "_id" : 4 }
+
+      Documents with ``_id`` equal to ``3`` and ``_id`` equal to ``4``
+      do not contain the ``students`` field since no element matched
+      the :projection:`$elemMatch` criteria.
+
+   When the :method:`~db.collection.find()` method includes a
+   :method:`~cursor.sort()`, the :method:`~db.collection.find()` method
+   applies the :method:`~cursor.sort()` to order the matching documents
+   **before** it applies the projection. 
+
+   If an array field contains multiple documents with the same field
+   name and the :method:`~db.collection.find()` method includes a
+   :method:`~cursor.sort()` on that repeating field, the returned
+   documents may not reflect the sort order because the
+   :method:`~cursor.sort()` was applied to the elements of the array
+   before the :projection:`elemMatch` projection.
+
+   .. example::
+
+      The following query includes a :method:`~cursor.sort()` to order
+      by descending ``students.age`` field:
+
+      .. code-block:: javascript
+
+         db.schools.find( { zipcode: 63109 }, { students: { $elemMatch: { school: 102 } } } ).sort( { "students.age": -1 } )
+
+      The operation applies the :method:`~cursor.sort()` to order the
+      documents that have the field ``zipcode`` equal to ``63109`` and
+      then applies the projection. The operation returns the three
+      documents in the following order:
 
       .. code-block:: javascript
 
-         {
-          dependents: [
-                       { name: "john", school: 102, age: 10 }
-                      ]
-         }
+         { "_id" : 1, "students" : [ { "name" : "john", "school" : 102, "age" : 10 } ] }
+         { "_id" : 3 }
+         { "_id" : 4, "students" : [ { "name" : "barney", "school" : 102, "age" : 7 } ] }
 
-.. note::
+.. seealso::
 
-   The :projection:`$elemMatch` projection will only match one array
-   element per source document.
+   :doc:`positional $ </reference/projection/positional>` projection operator