mongoengine/docs/tutorial.rst

Tutorial
========
This tutorial introduces **MongoEngine** by means of example --- we will walk
through how to create a simple **Tumblelog** application. A Tumblelog is a type
of blog where posts are not constrained to being conventional text-based posts.
As well as text-based entries, users may post images, links, videos, etc. For
simplicity's sake, we'll stick to text, image and link entries in our
application. As the purpose of this tutorial is to introduce MongoEngine, we'll
focus on the data-modelling side of the application, leaving out a user
interface.

Connecting to MongoDB
---------------------
Before we start, you should make sure that you have a copy of MongoDB running
in an accessible location --- running it locally will be easier, but if that is
not an option then it may be run on a remote server.

Before we can start using MongoEngine, we need to tell it how to connect to our
instance of **mongod**. For this we use the :func:`mongoengine.connect`
function. The only argument we need to provide is the name of the MongoDB
database to use::

    from mongoengine import *
    
    connect('tumblelog')

This will connect to a mongod instance running locally on the default port. To 
connect to a mongod instance running elsewhere, we may specify the host and
port explicitly::

    connect('tumblelog', host='192.168.1.35', port=12345)

Defining our documents
----------------------
MongoDB is *schemaless*, which means that no schema is enforced by the database
--- we may add and remove fields however we want and MongoDB won't complain.
This makes life a lot easier in many regards, especially when it comes to
migrations. However, defining schemata for our documents can help to iron out
bugs involving incorrect types or missing fields, and also allow us to define
utility methods on our documents in the same way that traditional :abbr:`ORMs
(Object-Relational Mappers)` do.

In our Tumblelog application we need to store several different types of
information.  We will need to have a collection of **users**, so that we may
link posts to an individual. We also need to store our different types
**posts** (text, image and link) in the database. For to aid navigation of our
Tumblelog, posts may have **tags** associated with them, so that the list of
posts shown to the user may be limited to posts that have a specified tag.
Finally, it would be nice if **comments** could be added to posts. We'll start
with **users**, as the others are slightly more involved.

Users
^^^^^
Just as if we were using a relational database with an ORM, we need to define
which fields a :class:`User` may have, and what their types will be::

    class User(Document):
        email = StringField(required=True)
        first_name = StringField(max_length=50)
        last_name = StringField(max_length=50)

This looks similar to how a the structure of a table would be defined in a
regular ORM. The key difference is that this schema will never be passed on to
MongoDB --- this will only be enforced at the application level. Also, the User
documents will be stored in a MongoDB *collection* rather than a table.

Posts, Comments and Tags
^^^^^^^^^^^^^^^^^^^^^^^^
Well that wasn't too bad, was it? Now we'll think about how to store the rest
of the information. If we were using a relational database, we would most
likely have a table of **posts**, a table of **comments** and a table of
**tags**.  To associate the comments with individual posts, we would put a
column in the comments table that contained a foreign key to the posts table.
We'd also need a link table to provide the many-to-many relationship between
posts and tags. Then we'd need to address the problem of storing the
specialised post-types (text, image and link). There are several ways we can
achieve this, but each of them have their problems --- none of them stand out
as particularly intuitive solutions.

Posts
"""""
But MongoDB *isn't* a relational database, so we're not going to do it that
way. As it turns out, we can use MongoDB's schemaless nature to provide us with
a much nicer solution. We will store all of the posts in *one collection* ---
each post type will just have the fields it needs. If we later want to add
video posts, we don't have to modify the collection at all, we just *start
using* the new fields we need to support video posts. This fits with the
Object-Oriented principle of *inheritance* nicely. We can think of
:class:`Post` as an base class, and :class:`TextPost`, :class:`ImagePost` and
:class:`LinkPost` as subclasses of :class:`Post`. In fact, MongoEngine supports
this kind of modelling out of the box::

    class Post(Document):
        title = StringField(max_length=120, required=True)
        author = ReferenceField(User)

    class TextPost(Post):
        content = StringField()

    class ImagePost(Post):
        image_path = StringField()

    class LinkPost(Post):
        link_url = StringField()

We are storing a reference to the author of the posts using a
:class:`mongoengine.ReferenceField` object. These are similar to foreign key
fields in traditional ORMs, and are automatically translated into references
when they are saved, and dereferenced when they are loaded.

Tags
""""
Now that we have our Post models figured out, how will we attach tags to them?
MongoDB allows us to store lists of items natively, so rather than having a
link table, we can just store a list of tags in each post. Also, for both
efficiency and simplicity's sake, we'll store the tags as strings directly
within the post, rather than storing references to tags in a separate
collection. Especially as tags are generally very short (often even shorted
than a document's id), this denormalisation won't impact very strongly on the 
size of our database. So let's take a look that the code our modified
:class:`Post` class::

    class Post(Document):
        title = StringField(max_length=120, required=True)
        author = ReferenceField(User)
        tags = ListField(StringField(max_length=30))

The :class:`mongoengine.ListField` object that is used to define a Post's tags
takes a field object as its first argument --- this means that you can have
lists of any type of field (including lists). Note that we don't need to
modify the specialised post types as they all inherit from :class:`Post`.

Comments
""""""""
A comment is typically associated with *one* post. In a relational database, to
display a post with its comments, we would have to retrieve the post from the
database, then query the database again for the comments associated with the
post. This works, but there is no real reason to be storing the comments
separately from their associated posts, other than to work around the
relational model. Using MongoDB we can store the comments as a list of
*embedded documents* directly on the post document. An embedded document should
be treated no differently that a regular document; it just doesn't have its own
collection. Using MongoEngine, we can define the structure of embedded
documents, along with utility methods, in exactly the same way we do with
regular documents::

    class Comment(EmbeddedDocument):
        content = StringField()
        name = StringField(max_length=120)

We can then store a list of comment documents in our post document::

    class Post(Document):
        title = StringField(max_length=120, required=True)
        author = ReferenceField(User)
        tags = ListField(StringField(max_length=30))
        comments = ListField(EmbeddedDocumentField(Comment))

Adding data to our Tumblelog
----------------------------
Now that we've defined how our documents will be structured, lets start adding
some documents to the database. Firstly, we'll need to create a :class:`User`
object::

    john = User(email='jdoe@example.com', first_name='John', last_name='Doe')
    john.save()

Simple, eh? Note that only fields with ``required=True`` need to be specified
in the constructor, we could have also defined our user using attribute
syntax::

    john = User(email='jdoe@example.com')
    john.first_name = 'John'
    john.last_name = 'Doe'
    john.save()

Now that we've got our user in the database, lets add a couple of posts::

    post1 = TextPost(title='Fun with MongoEngine', author=john)
    post1.content = 'Took a look at MongoEngine today, looks pretty cool.'
    post1.tags = ['mongodb', 'mongoengine']
    post1.save()

    post2 = LinkPost(title='MongoEngine Documentation', author=john)
    post2.link_url = 'http://tractiondigital.com/labs/mongoengine/docs'
    post2.tags = ['mongoengine']
    post2.save()

Note that if you change a field on a object that has already been saved, then
call :meth:`save` again, the document will be updated.

Accessing our data
------------------
So now we've got a couple of posts in our database, how do we display them?
Each document class (i.e. any class that inherits either directly or indirectly
from :class:`mongoengine.Document`) has an :attr:`objects` attribute, which is
used to access the documents in the database associated with that class. So
lets see how we can get our posts' titles::

    for post in Post.objects:
        print post.title

Retrieving type-specific information
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This will print the titles of our posts, one on each line. But What if we want
to access the type-specific data (link_url, content, etc.)? One way is simply
to use the :attr:`objects` attribute of a subclass of :class:`Post`::

    for post in TextPost.objects:
        print post.content

Using TextPost's :attr:`objects` attribute only returns documents that were
created using :class:`TextPost`. Actually, there is a more general rule here:
the :attr:`objects` attribute of any subclass of :class:`mongoengine.Document`
only looks for documents that were created using that subclass or one of its
subclasses.

So how would we display all of our posts, showing only the information that
corresponds to each post's specific type? As you might have guessed, there is a
better way than just using each of the subclasses individually. When we used
:class:`Post`'s :attr:`objects` attribute earlier, the objects being returned
weren't actually instances of :class:`Post` --- they were instances of the
subclass of :class:`Post` that matches the post's type. Lets look at how this
works in practice::

    for post in Post.objects:
        print post.title
        print '=' * len(post.title)

        if isinstance(post, TextPost):
            print post.content

        if isinstance(post, LinkPost):
            print 'Link:', post.link_url

        print

This would print the title of each post, followed by the content if it was a
text post, and "Link: <url>" if it was a link post.

Searching our posts by tag
^^^^^^^^^^^^^^^^^^^^^^^^^^
The :attr:`objects` attribute of a :class:`mongoengine.Document` is actually a
:class:`mongoengine.QuerySet` object. This lazily queries the database only
when you need the data. It may also be filtered to narrow down your query. Lets
adjust our query so that only posts with the tag "mongodb" are returned::

    for post in Post.objects(tags='mongodb'):
        print post.title

There are also methods available on :class:`mongoengine.QuerySet` objects that
allow different results to be returned, for example, calling :meth:`first` on
the :attr:`objects` attribute will return a single document, the first matched
by the query you provide. Aggregation functions may also be used on
:class:`mongoengine.QuerySet` objects::

    num_posts = Post.objects(tags='mongodb').count()
    print 'Found % posts with tag "mongodb"' % num_posts