Fundamentals of Testing Using Rails/

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Using Mocks to Simulate Database Failure

Using Mocks to Simulate Database Failure

Learn how to simulate database failure using mocks.

We'll cover the following...

Simulating database failure

A common use case for test doubles in Rails is to simulate database failures. Let’s look at how we can do that.

In the past, we would have suggested testing for failure at the controller level. However, Rails has basically deprecated controller tests for reasons we’ll talk about in Testing Rails Display Elements, and so we’d like to stay within the bounds of Rails community behavior.

Let’s look at the project-creation functionality we wrote at the beginning of the book. We wrote a controller:

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def create
    @workflow = CreatesProject.new(
    name: params[:project][:name],
    task_string: params[:project][:tasks])
    @workflow.create
    if @workflow.success?
        redirect_to projects_path
    else
        @project = @workflow.project
        render :new
    end
    
end

Workflow

The controller largely defers to a workflow object that does the actual work:

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class CreatesProject
  attr_accessor :name, :project, :task_string
  def initialize(name: "", task_string: "")
    @name = name
    @task_string = task_string || ""
    @success = false
  end
  def success?
    @success
  end
  def build
    self.project = Project.new(name: name)
    project.tasks = convert_string_to_tasks
    project
  end
  def create
    build
    result = project.save
    @success = result
  end
  def convert_string_to_tasks
    task_string.split("\n").map do |one_task|
      title, size_string = one_task.split(":")
      Task.new(title: title, size: size_as_integer(size_string))
    end
  end
  def size_as_integer(size_string)
    return 1 if size_string.blank?
    [size_string.to_i, 1].max
  end
end

Now we have two different pieces of point-of-failure logic to deal with:

  • In the event of a database failure, the ...