Time for action — changing models

Let's take a look at the Book model: 

class Book
include Mongoid::Document
field :title, type: String
field :publisher, type: String
end

If we have such a model, what does the object look like? Execute the following command to find out:

irb> Book.create(publisher: "Dover")
=> #<Book _id: 4f216427fed0eb86ac000001, _type: nil, title: nil, publisher: "Dover">

Now, suppose we wanted to add a few fields to the Book model, how do we do that? Change the code! The code would now look like the following:

class Book
include Mongoid::Document
field :title, type: String
field :publisher, type: String
field :published_on, type: Date
end

What just happened?

Now, let's see what happens when we create a new object as well as access the earlier one we created. Execute the following commands:

irb> Book.create(publisher: "Packt", published_on: Date.today)
=> #<Book _id: 4f21660cfed0eb86ac000002, _type: nil, title: nil, publisher: "Packt", published_on: 2012-01-26 00:00:00 UTC>

So far, so good! But what happens to the earlier object created?

irb> Book.where(publisher: "Dover").first
=> #<Book _id: 4f216427fed0eb86ac000001, _type: nil, title: nil, publisher: "Dover", published_on: nil>

Notice the published_on field that is nil!

Tip

Always try to avoid removing fields — it can cause undue trouble.

So, go forth and change the models to your heart's content! No worries.

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