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docs/guide/querying.rst

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+=====================
+Querying the database
+=====================
+:class:`~mongoengine.Document` classes have an :attr:`objects` attribute, which
+is used for accessing the objects in the database associated with the class.
+The :attr:`objects` attribute is actually a
+:class:`~mongoengine.queryset.QuerySetManager`, which creates and returns a new
+a new :class:`~mongoengine.queryset.QuerySet` object on access. The
+:class:`~mongoengine.queryset.QuerySet` object may may be iterated over to
+fetch documents from the database::
+
+    # Prints out the names of all the users in the database
+    for user in User.objects:
+        print user.name
+
+Filtering queries
+-----------------
+The query may be filtered by calling the
+:class:`~mongoengine.queryset.QuerySet` object with field lookup keyword 
+arguments. The keys in the keyword arguments correspond to fields on the
+:class:`~mongoengine.Document` you are querying::
+
+    # This will return a QuerySet that will only iterate over users whose
+    # 'country' field is set to 'uk'
+    uk_users = User.objects(country='uk')
+
+Fields on embedded documents may also be referred to using field lookup syntax
+by using a double-underscore in place of the dot in object attribute access
+syntax::
+    
+    # This will return a QuerySet that will only iterate over pages that have
+    # been written by a user whose 'country' field is set to 'uk'
+    uk_pages = Page.objects(author__country='uk')
+
+Querying lists
+^^^^^^^^^^^^^^
+On most fields, this syntax will look up documents where the field specified
+matches the given value exactly, but when the field refers to a
+:class:`~mongoengine.ListField`, a single item may be provided, in which case
+lists that contain that item will be matched::
+
+    class Page(Document):
+        tags = ListField(StringField())
+
+    # This will match all pages that have the word 'coding' as an item in the
+    # 'tags' list
+    Page.objects(tags='coding')
+
+Query operators
+---------------
+Operators other than equality may also be used in queries; just attach the
+operator name to a key with a double-underscore::
+    
+    # Only find users whose age is 18 or less
+    young_users = Users.objects(age__lte=18)
+
+Available operators are as follows:
+
+* ``neq`` -- not equal to
+* ``lt`` -- less than
+* ``lte`` -- less than or equal to
+* ``gt`` -- greater than
+* ``gte`` -- greater than or equal to
+* ``in`` -- value is in list (a list of values should be provided)
+* ``nin`` -- value is not in list (a list of values should be provided)
+* ``mod`` -- ``value % x == y``, where ``x`` and ``y`` are two provided values
+* ``all`` -- every item in array is in list of values provided
+* ``size`` -- the size of the array is 
+* ``exists`` -- value for field exists
+
+Limiting and skipping results
+-----------------------------
+Just as with traditional ORMs, you may limit the number of results returned, or
+skip a number or results in you query.
+:meth:`~mongoengine.queryset.QuerySet.limit` and
+:meth:`~mongoengine.queryset.QuerySet.skip` and methods are available on
+:class:`~mongoengine.queryset.QuerySet` objects, but the prefered syntax for
+achieving this is using array-slicing syntax::
+
+    # Only the first 5 people
+    users = User.objects[:5]
+
+    # All except for the first 5 people
+    users = User.objects[5:]
+
+    # 5 users, starting from the 10th user found
+    users = User.objects[10:15]
+
+Aggregation
+-----------
+MongoDB provides some aggregation methods out of the box, but there are not as
+many as you typically get with an RDBMS. MongoEngine provides a wrapper around
+the built-in methods and provides some of its own, which are implemented as
+Javascript code that is executed on the database server.
+
+Counting results
+^^^^^^^^^^^^^^^^
+Just as with limiting and skipping results, there is a method on
+:class:`~mongoengine.queryset.QuerySet` objects -- 
+:meth:`~mongoengine.queryset.QuerySet.count`, but there is also a more Pythonic
+way of achieving this::
+
+    num_users = len(User.objects)
+
+Further aggregation
+^^^^^^^^^^^^^^^^^^^
+You may sum over the values of a specific field on documents using
+:meth:`~mongoengine.queryset.QuerySet.sum`::
+
+    yearly_expense = Employee.objects.sum('salary')
+
+.. note::
+   If the field isn't present on a document, that document will be ignored from
+   the sum.
+
+To get the average (mean) of a field on a collection of documents, use
+:meth:`~mongoengine.queryset.QuerySet.average`::
+
+    mean_age = User.objects.average('age')
+
+As MongoDB provides native lists, MongoEngine provides a helper method to get a
+dictionary of the frequencies of items in lists across an entire collection --
+:meth:`~mongoengine.queryset.QuerySet.item_frequencies`. An example of its use
+would be generating "tag-clouds"::
+
+    class Article(Document):
+        tag = ListField(StringField())
+
+    # After adding some tagged articles...
+    tag_freqs = Article.objects.item_frequencies('tag', normalize=True)
+
+    from operator import itemgetter
+    top_tags = sorted(tag_freqs.items(), key=itemgetter(1), reverse=True)[:10]
+
+Advanced queries
+----------------
+Sometimes calling a :class:`~mongoengine.queryset.QuerySet` object with keyword
+arguments can't fully express the query you want to use -- for example if you
+need to combine a number of constraints using *and* and *or*. This is made 
+possible in MongoEngine through the :class:`~mongoengine.queryset.Q` class.
+A :class:`~mongoengine.queryset.Q` object represents part of a query, and
+can be initialised using the same keyword-argument syntax you use to query
+documents. To build a complex query, you may combine 
+:class:`~mongoengine.queryset.Q` objects using the ``&`` (and) and ``|`` (or)
+operators. To use :class:`~mongoengine.queryset.Q` objects, pass them in
+as positional arguments to :attr:`Document.objects` when you filter it by
+calling it with keyword arguments::
+
+    # Get published posts
+    Post.objects(Q(published=True) | Q(publish_date__lte=datetime.now()))
+
+    # Get top posts
+    Post.objects((Q(featured=True) & Q(hits__gte=1000)) | Q(hits__gte=5000))
+
+.. warning::
+   Only use these advanced queries if absolutely necessary as they will execute
+   significantly slower than regular queries. This is because they are not
+   natively supported by MongoDB -- they are compiled to Javascript and sent
+   to the server for execution.
+
+.. _guide-atomic-updates:
+
+Atomic updates
+--------------
+Documents may be updated atomically by using the
+:meth:`~mongoengine.queryset.QuerySet.update_one` and
+:meth:`~mongoengine.queryset.QuerySet.update` methods on a 
+:meth:`~mongoengine.queryset.QuerySet`. There are several different "modifiers"
+that you may use with these methods:
+
+* ``set`` -- set a particular value
+* ``unset`` -- delete a particular value (since MongoDB v1.3+)
+* ``inc`` -- increment a value by a given amount
+* ``dec`` -- decrement a value by a given amount
+* ``push`` -- append a value to a list
+* ``push_all`` -- append several values to a list
+* ``pull`` -- remove a value from a list
+* ``pull_all`` -- remove several values from a list
+
+The syntax for atomic updates is similar to the querying syntax, but the 
+modifier comes before the field, not after it::
+    
+    >>> post = BlogPost(title='Test', page_views=0, tags=['database'])
+    >>> post.save()
+    >>> BlogPost.objects(id=post.id).update_one(inc__page_views=1)
+    >>> post.reload()  # the document has been changed, so we need to reload it
+    >>> post.page_views
+    1
+    >>> BlogPost.objects(id=post.id).update_one(set__title='Example Post')
+    >>> post.reload()
+    >>> post.title
+    'Example Post'
+    >>> BlogPost.objects(id=post.id).update_one(push__tags='nosql')
+    >>> post.reload()
+    >>> post.tags
+    ['database', 'nosql']