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Updated tutorial, added tutorial link to readme

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Harry Marr 2009-12-16 18:47:53 +00:00
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README.rst
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@ -2,5 +2,7 @@ MongoEngine
===========
MongoEngine is an ORM-like layer on top of PyMongo.
Tutorial available at http://hmarr.com/mongoengine/
**Warning:** this software is still in development and should *not* be used
in production.

90
docs/tutorial.rst
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@ -11,9 +11,9 @@ 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 start, make sure that a copy of MongoDB is 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`
@ -25,8 +25,8 @@ database to use::
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 to a mongod instance running elsewhere, specify the host and port
explicitly::
connect('tumblelog', host='192.168.1.35', port=12345)
@ -34,20 +34,20 @@ 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.
This makes life a lot easier in many regards, especially when there is a change
to the data model. 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
**posts** (text, image and link) in the database. 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.
posts shown to the user may be limited to posts that have been assigned 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
^^^^^
@ -66,16 +66,15 @@ 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.
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
"""""
@ -86,7 +85,7 @@ 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:`Post` as a 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::
@ -112,10 +111,10 @@ 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
link table, we can just store a list of tags in each post. So, 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
collection. Especially as tags are generally very short (often even shorter
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::
@ -138,11 +137,11 @@ 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
*embedded documents* directly on a 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::
collection in the database. 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()
@ -158,23 +157,22 @@ We can then store a list of comment documents in our post document::
Adding data to our Tumblelog
----------------------------
Now that we've defined how our documents will be structured, lets start adding
Now that we've defined how our documents will be structured, let's 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::
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::
Now that we've got our user in the database, let's add a couple of posts::
post1 = TextPost(title='Fun with MongoEngine', author=john)
post1.content = 'Took a look at MongoEngine today, looks pretty cool.'
@ -194,8 +192,8 @@ 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::
used to access the documents in the database collection associated with that
class. So let's see how we can get our posts' titles::
for post in Post.objects:
print post.title
@ -216,12 +214,12 @@ 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::
corresponds to each post's specific type? 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. Let's look at how this works in
practice::
for post in Post.objects:
print post.title
@ -242,8 +240,8 @@ 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::
when you need the data. It may also be filtered to narrow down your query.
Let's adjust our query so that only posts with the tag "mongodb" are returned::
for post in Post.objects(tags='mongodb'):
print post.title