I wanted to export a MongoMapper document and it’s related documents as JSON — with embedded arrays for the collections. Invoking to_json did not seem to work perfectly, so I set about to discover what was going on.

Conclusion

If you use Embedded Documents for every associated document, the to_json method will work perfectly.

If you have normal Documents, you must override the as_json method to export the object “tree.”

Details

Here is a walk through of exporting mongo documents as JSON.

I created a simple Author class. And will use a simple test to show how to_json works:

describe "Author 1" do
  before :all do
    class Author
      include MongoMapper::Document
      key :name
      key :pen_name
    end
  end

  it "should output JSON" do
    p1 = Author.create(:name => "Ben Franklin", :pen_name => "Poor Richard")
    json = p1.to_json
    puts json
    json.should include "name"
    json.should include "Poor Richard"
  end
end

And we get what we expect:

{
  "books":[],
  "id":"4f316a4c8951a2eefe000001", 
  "name":"Ben Franklin",
  "pen_name":"Poor Richard"
}

Now let’s add a new Book document of the Embedded variety. Here we will assert that the Author JSON should include a list of Books:

describe "Author 2" do
  before :all do
    class Book
      include MongoMapper::EmbeddedDocument
      key :title
    end
    class Author
      include MongoMapper::Document
      many :books
    end
  end
  it "authors have books" do
    p1 = Author.create(:name => "Ben Franklin", :pen_name => "Poor Richard",
                       :books => [Book.new(:title => "Poor Richard's Almanac")])
    json = p1.to_json
    puts json
    json.should include "Poor Richard"
    json.should include "Almanac"
  end
end

And, sure enough, it works.

{
  "books":[{
    "id":"4f316a4c8951a2eefe000003", 
    "title":"Poor Richard's Almanac"}],
  "id":"4f316a4c8951a2eefe000004", 
  "name":"Ben Franklin", 
  "pen_name":"Poor Richard"
}

Let’s add a list of Interests to the Author class, this time as a normal document type (not embedded). Now we can test that the Author JSON has the expected Interest:

describe "Author 3" do
  before :all do
    class Interest
      include MongoMapper::Document
      key :title, String
    end
    class Book
      include MongoMapper::EmbeddedDocument
      key :title
    end
    class Author
      include MongoMapper::Document
      many :books
      many :interests
    end
  end
  it "should have interests" do
    p1 = Author.create(:name => "Ben Franklin", :pen_name => "Poor Richard",
                       :books => [Book.new(:title => "Poor Richard's Almanac")],
                       :interests => [Interest.create(:title => "Movies")])
    json = p1.to_json
    puts json
    json.should include "Poor Richard"
    json.should include "Almanac"
    json.should include "Movies" # Fails
  end
end

Whoa! No joy! Seems that the association to non-Embedded documents does not get automatically exported to the JSON.

{
  "books":[{
    "id":"4f316a4c8951a2eefe000006",
    "title":"Poor Richard's Almanac"}],
  "id":"4f316a4c8951a2eefe000007", 
  "name":"Ben Franklin", 
  "pen_name":"Poor Richard"
}

And we get a failed spec 🙁

# expected "{"books":[{"id":"4f316a4c8951a2eefe000006","title":"Poor Richard's Almanac"}],"id":"4f316a4c8951a2eefe000007","name":"Ben Franklin","pen_name":"Poor Richard"}" to include "Movies"

Turns out we can add a custom as_json implementation to the class that you want to export as JSON. The as_json is responsible for indicating which fields and collections should be included in the json.

describe "Author 4" do
  before :all do
    class Interest
      include MongoMapper::Document
      key :title, String

    end
    class Book
      include MongoMapper::EmbeddedDocument
      key :title
    end
    class Author
      include MongoMapper::Document
      many :books
      many :interests

      def as_json options={}
        {
            :name => self.name,
            :pen_name => self.pen_name,
            :books => self.books,
            :interests => self.interests
        }
      end
    end
  end
  it "should have interests in json" do
    p1 = Author.create(:name => "Ben Franklin", :pen_name => "Poor Richard",
                       :books => [Book.new(:title => "Poor Richard's Almanac")],
                       :interests => [Interest.create(:title => "Movies")])
    json = p1.to_json
    puts json
    json.should include "Poor Richard"
    json.should include "Almanac"
    json.should include "Movies"
  end
end

And we have Books and Interests. Success!

{
  "name":"Ben Franklin", 
  "pen_name":"Poor Richard",
  "books":[{
    "id":"4f31782a8951a2f267000002", 
    "title":"Poor Richard's Almanac"}], 
  "interests":[{
    "author_id":"4f31782a8951a2f267000003", 
    "id":"4f31782a8951a2f267000001",
    "title":"Movies"}]
}