Simple example of using MongoMapper with Ruby

The MongoMapper web site is really lacking on even simple examples, especially those that don’t use Rails. So, here’s a simple example that might help someone.

From the Gemfile:

source ''

gem 'mongo_mapper'

And then the application:

require 'mongo_mapper'

# jam it into the database "mm"
MongoMapper.database = "mm"

class App

def create_user
user = => 'Johnny')
puts "user created"!

def find_user
query = User.where(:name => 'Johnny')
user = query.first # just the first
if not user.nil?

def delete_user
query = User.where(:name => 'Johnny')
user = query.first # just the first


class User
include MongoMapper::Document
key :name, String

app =

The code does a few things:

  1. Creates a new user with a single field called name.
  2. Finds the user using the where function
  3. Removes (destroys/deletes) the user

The key things to note are that the where function returns a query and not the actual results. The results are fetched on demand. This is very similar to the extension methods and LINQ in .NET as those functions build a query that executed only when the results are first requested.

The same thing is true of MongoMapper in this case. The results are not returned until the first function is called. Alternatively, all or last could have been used. all of course returning a list of results that could be iterated in a loop.

If there were no results, the result of calling first in this example would be that the user variable would be nil.

The delete_user function above has absolutely no error checking.

Ember.JS and EZdata, and Rails

I’ve been trying to do some additional work on my ember.js extension for data management. At the same time though, I’ve been trying (to learn and) build a simple Ruby on Rails web demo application using the new JavaScript library. There have been more than a few things that have mystified me about the framework and the structuring of an application. One aspect in particular was how to best manage foreign keys and join tables with the ActiveRecord class (and the corresponding SQL tables). So many tutorials have the same lame example of: a CART, an ORDER, a CUSTOMER …, that it’s often difficult to apply the same patterns to a more interesting system.

I started simple this time.

I wanted a PERSON class and a GIFT class.


A Person has been given gifts and may give gifts (and a few other common attributes).

class CreatePeople < ActiveRecord::Migration
  def change
    create_table :people do |t|
      t.string :first_name
      t.string :last_name :date_of_birth
      t.string :email_address


One of the things that I can’t decide if I like is the automatic pluralization of words, especially People/Person. I would have been content with a Persons table, but when creating a model, by default (as I’m aware it can be overridden), a Person is mapped to a table called “People.”

The second table, Gifts is very simple:

class CreateGifts < ActiveRecord::Migration
  def change
    create_table :gifts do |t|
      t.string :description
      t.integer :from_person_id
      t.integer :to_person_id

As I thought I might want a richer structure for the Gift class in the future, I did not use the more standard “person_id” name for the foreign key column that maps a gift to a Person. I wanted the column name to be more obvious what it was. Additionally, I needed two columns that both mapped to a “Person", so I couldn’t have both be called “person_id” anyway.

By deviating from the normal pattern, there are a few expectations when defining the ActiveRecord class. It was these expectations that weren’t clear to me (especially with examples).

The Ruby class for Gift is defined like so:

class Gift < ActiveRecord::Base
  belongs_to :from_person, :class_name => "Person", :foreign_key => "from_person_id"
  belongs_to :to_person, :class_name => "Person", :foreign_key => "to_person_id"

and the Person:

class Person < ActiveRecord::Base
  has_many  :gifts_given , :class_name => "Gift", :foreign_key => "from_person_id"
  has_many  :gifts, :foreign_key => "to_person_id"

The key (and the ‘ah ha’ moment for me) was the use of the foreign_key parameter to the on the has_many and belongs_to associations.

In the Gift class, I included the belongs_to association macro. In this case, :from_person is the name of the rich accessor method (which looks like a property in other languages) that will be added to the Gift class. Using the symbol :class_name is like a class finding assistant. Without it, the Rails framework assumes that there would be a class named “FromPerson.” Of course, that would fail. By specifying “Person,” I’ve indicated to Rails that the class it should map to is called “Person” which I defined earlier. The :foreign_key symbol and value indicates which column in the backing table has the value which will map to an instance of a Person. In the SQL table, I added a “from_person_id” column and this points at that as the “from_person_id” column is the foreign key to the People table. (The same is true for :to_person.)

Looking at the Person class, it is using another common association macro, :has_many. :Has_many when used here, is indicating that a Person may have zero or more “gifts.” The new accessor method is named gifts (by using :gifts). Here, too, you’ll specify the name of the foreign_key. Again, repeat this for the :gifts_given automatically added accessor method. One interesting thing is that only :gifts_given requires the :class_name to be specified. The reason is that Rails automatically maps :gifts to the Gifts class (by way of naming). The :gifts_given cannot be automatically mapped, so you need to give (sigh) it a little help.

Here’s a little test:

>> jason = Person.find(1)
  Person Load (18.0ms)  SELECT "people".* FROM "people" WHERE "people"."id" = ? LIMIT 1  [["id", 1]]
#<Person id: 1, first_name: "Jason", last_name: "Bourne", date_of_birth: nil, email_address: nil, created_at: "2012-01-06 13:47:40", updated_at: "2012-01-07 03:10:29">
>> jason.gifts_given
  Gift Load (0.0ms)  SELECT "gifts".* FROM "gifts" WHERE "gifts"."from_person_id" = 1
[#<Gift id: 1, description: "Machine Gun", from_person_id: 1, to_person_id: 4, created_at: "2012-01-07 14:39:17", updated_at: "2012-01-07 14:39:17">]
>> jason.gifts_given[0].to_person.first_name
  Person Load (0.0ms)  SELECT "people".* FROM "people" WHERE "people"."id" = 4 LIMIT 1
>> jason.gifts_given[0]
  [#<Gift id: Gift Load (1.0ms)1  SELECT "gifts".* FROM "gifts" WHERE "gifts"."to_person_id" = 4
, description: "Machine Gun", from_person_id: 1, to_person_id: 4, created_at: "2012-01-07 14:39:17", updated_at: "2012-01-07 14:39:17">]
>> jason.gifts_given[0][0].from_person.first_name
  Person Load (0.0ms)  SELECT "people".* FROM "people" WHERE "people"."id" = 1 LIMIT 1

I added two people: Jason, and Magnum, and one gift before executing the code above. Jason, as you should be able to follow, gave a wonderful gift to Magnum. As you can see, by using the automatically added accessor methods by way of the association macros described above, I was able to traverse the database structure very easily when mapped to a few simple objects.

One plus of experimenting and testing with the console while using Rails/Ruby in this case is that the output includes the SQL commands that are executed when the various method calls are made. Here’s an example where I rolled multiple calls into one chained call:

>> jason = Person.find(1).gifts_given[0].to_person.first_name
  Person Load (19.0ms)  SELECT "people".* FROM "people" WHERE "people"."id" = ? LIMIT 1  [["id", 1]]
  Gift Load (0.0ms)  SELECT "gifts".* FROM "gifts" WHERE "gifts"."from_person_id" = 1
  Person Load (0.0ms)  SELECT "people".* FROM "people" WHERE "people"."id" = 4 LIMIT 1