Getting started with DataMapper

First, if you think you might need some help, there’s an active community supporting DataMapper through the mailing list and the #datamapper IRC channel on irc.freenode.net.

So lets imagine we’re setting up some models for a blogging app. We’ll keep it nice and simple. The first thing to decide on is what models we want. Post is a given. So is Comment. But let’s mix it up and do Category too.

Install DataMapper

If you have RubyGems installed, pop open your console and install a few things.

gem install dm-core

If you are planning on using DataMapper with a database, install a database driver from the DataObjects project: (Substitute dm-sqlite-adapter with dm-postgres-adapter or dm-mysql-adapter depending on your preferences)

gem install dm-sqlite-adapter

Require it in your application

require 'rubygems'
require 'dm-core'

Specify your database connection

You need make sure this is set before you define your models.

  # If you want the logs displayed you have to do this before the call to setup
  DataMapper::Logger.new($stdout, :debug)

  # An in-memory Sqlite3 connection:
  DataMapper.setup(:default, 'sqlite::memory:')

  # A MySQL connection:
  DataMapper.setup(:default, 'mysql://localhost/the_database_name')

  # A Postgres connection:
  DataMapper.setup(:default, 'postgres://localhost/the_database_name')

Define your models

The Post model is going to need to be persistent, so we’ll include DataMapper::Resource. The convention with model names is to use the singular, not plural version…but that’s just the convention, you can do whatever you want.

class Post
  include DataMapper::Resource

  property :id,         Serial   # An auto-increment integer key
  property :title,      String   # A varchar type string, for short strings
  property :body,       Text     # A text block, for longer string data.
  property :created_at, DateTime # A DateTime, for any date you might like.
end

class Comment
  include DataMapper::Resource

  property :id,         Serial
  property :posted_by,  String
  property :email,      String
  property :url,        String
  property :body,       Text
end

class Category
  include DataMapper::Resource

  property :id,         Serial
  property :name,       String
end

The above example is simplified, but you can also specify more options such as constraints for your properties. DataMapper supports a lot of different property types natively, and more through dm-types.

Associations

Ideally, these declarations should be done inside your class definition with the properties and things, but for demonstration purposes, we’re just going to crack open the classes.

One To Many

Posts can have comments, so we’ll need to setup a simple one-to-many association between then:

class Post
  has n, :comments
end

class Comment
  belongs_to :post
end

Has and belongs to many

Has and belongs to many Categories can have many Posts and Posts can have many Categories, so we’ll need a many to many relationships commonly referred to “has and belongs to many”. We’ll setup a quick model to wrap our join table between the two so that we can record a little bit of meta-data about when the post was categorized into a category.

class Categorization
  include DataMapper::Resource

  property :id,         Serial
  property :created_at, DateTime

  belongs_to :category
  belongs_to :post
end

# Now we re-open our Post and Categories classes to define associations
class Post
  has n, :categorizations
  has n, :categories, :through => :categorizations
end

class Category
  has n, :categorizations
  has n, :posts,      :through => :categorizations
end

Finalize Models

After declaring all of the models, you should finalize them:

DataMapper.finalize

This checks the models for validity and initializes all properties associated with relationships. It is likely if you use a web-framework such as merb or rails, this will already be done for you. In case you do not, be sure to call it at an appropriate time.

DataMapper allows the use of natural primary keys, composite primary keys and other complexities. Because of this, when a model is declared with a belongs_to relationship the property to hold the foreign key cannot be initialized immediately. It can only be initialized when the parent model has also been declared. This is hard for DataMapper to determine, due to the dynamic nature of ruby, so it is left up to developers to determine the appropriate time.

Set up your database tables

Relational Databases work with pre-defined tables. To be able to create the tables in the underlying storage, you need to require dm-migrations first.

require  'dm-migrations'

Once you made sure that dm-migrations is available, you can create the tables by issuing the following command.

DataMapper.auto_migrate!

This will issue the necessary CREATE statements (DROPing the table first, if it exists) to define each storage according to their properties. After auto_migrate! has been run, the database should be in a pristine state. All the tables will be empty and match the model definitions.

This wipes out existing data, so you could also do:

DataMapper.auto_upgrade!

This tries to make the schema match the model. It will CREATE new tables, and add columns to existing tables. It won’t change any existing columns though (say, to add a NOT NULL constraint) and it doesn’t drop any columns. Both these commands also can be used on an individual model (e.g. Post.auto_migrate!)

Create your first resource

Using DataMapper to create a resource (A resource is an instance of a model) is simple

# create makes the resource immediately
@post = Post.create(
  :title      => "My first DataMapper post",
  :body       => "A lot of text ...",
  :created_at => Time.now
)

# Or new gives you it back unsaved, for more operations
@post = Post.new(:title => ..., ...)
@post.save                           # persist the resource

Both are equivalent. The first thing to notice is we didn’t specify the auto-increment key. This is because the data-store will provide that value for us, and should make sure it’s unique, too. Also, note that while the property is a DateTime, we can pass it a Time instance, and it will convert (or typecast) the value for us, before it saves it to the data-store. Any properties which are not specified in the hash will take their default values in the data-store.