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Getting started with DataMapper
Wed Aug 29 20:36:53 +0930 2007


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

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 an Adapter

First, you will need to install an Adapter, which allows DataMapper to communicate to the Database:

  • dm-sqlite-adapter

    # Debian / Ubuntu
    sudo apt-get install libsqlite3-dev
    # RedHat / Fedora
    sudo yum install sqlite-devel
    # MacPorts
    sudo port install sqlite3
    # HomeBrew
    sudo brew install sqlite
    gem install dm-sqlite-adapter
  • dm-mysql-adapter

    # Debian / Ubuntu
    sudo apt-get install libmysqlclient-dev
    # RedHat / Fedora
    sudo yum install mysql-devel
    # MacPorts
    sudo port install mysql5
    # HomeBrew
    sudo brew install mysql
    gem install dm-mysql-adapter
  • dm-postgres-adapter

    # Debian / Ubuntu
    sudo apt-get install libpq-dev
    # RedHat / Fedora
    sudo yum install postgresql-devel
    # MacPorts
    sudo port install postgresql91
    # HomeBrew
    sudo brew install postgresql
    gem install dm-postgres-adapter

Install DataMapper

If you have RubyGems installed, open a Terminal and install a few things.

gem install data_mapper

This will install the following, most commonly used DataMapper gems.

Require it in your application

{% highlight ruby %} require 'rubygems' require 'data_mapper' # requires all the gems listed above {% endhighlight %}

Specify your database connection

You need to make sure to do this before you use your models, i.e. before you actually start accessing the database.

{% highlight ruby %} # If you want the logs displayed you have to do this before the call to setup$stdout, :debug)

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

# A Sqlite3 connection to a persistent database DataMapper.setup(:default, 'sqlite:///path/to/project.db')

# A MySQL connection: DataMapper.setup(:default, 'mysql://user:password@hostname/database')

# A Postgres connection: DataMapper.setup(:default, 'postgres://user:password@hostname/database') {% endhighlight %}

Note: that currently you must setup a :default repository to work with DataMapper (and to be able to use additional differently named repositories). This might change in the future.

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.

{% highlight ruby %} 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 {% endhighlight %}

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.

An important thing to note is that every model must have a key in order to be valid. If a model has no key, there's no way to identify a resource and thus no way to update its persistent state within the backend datastore. DataMapper will raise a DataMapper::IncompleteModelError when trying to auto_migrate! a model that has no key declared.

Have a look at property to learn about the different ways of declaring keys for your models.


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

One To Many

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

{% highlight ruby %} class Post has n, :comments end

class Comment belongs_to :post end {% endhighlight %}

Has and belongs to many

Categories can have many Posts and Posts can have many Categories, so we’ll need a many to many relationship 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.

{% highlight ruby %}

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

{% endhighlight %}

Finalize Models

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

{% highlight ruby %} DataMapper.finalize {% endhighlight %}

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.

In general, you want to call finalize before your application starts accessing the models.

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 have dm-migrations loaded.

Note: If you've been following this instructions and did require 'data_mapper', you can safely skip the following require statement as it has already been done for you.

{% highlight ruby %} require 'dm-migrations' {% endhighlight %}

Once dm-migrations is loaded, you can create the tables by issuing the following command:

{% highlight ruby %} DataMapper.auto_migrate! {% endhighlight %}

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:

{% highlight ruby %} DataMapper.auto_upgrade! {% endhighlight %}

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

{% highlight ruby %}

create makes the resource immediately

@post = Post.create( :title => "My first DataMapper post", :body => "A lot of text ...", :created_at => )

Or new gives you it back unsaved, for more operations

@post = => ..., ...) # persist the resource {% endhighlight %}

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.