Question

I’m developing a RESTful API using Sinatra and DataMapper. When my models fail validation, I want to return JSON to indicate what fields were in error. DataMapper adds an ‘errors’ attribute to my model of type DataMapper::Validations::ValidationErrors. I want to return a JSON representation of this attribute.

Here’s a single file example (gotta love Ruby/Sinatra/DataMapper!):

require 'sinatra'
require 'data_mapper'
require 'json'


class Person
    include DataMapper::Resource

    property :id, Serial
    property :first_name, String, :required => true
    property :middle_name, String
    property :last_name, String, :required => true
end


DataMapper.setup :default, 'sqlite::memory:'
DataMapper.auto_migrate!


get '/person' do
    person = Person.new :first_name => 'Dave'
    if person.save
        person.to_json
    else
        # person.errors - what to do with this?
        { :errors => [:last_name => ['Last name must not be blank']] }.to_json
    end
end


Sinatra::Application.run!

In my actual app, I’m handling a POST or PUT, but to make the problem easy to reproduce, I’m using GET so you can use curl http://example.com:4567/person or your browser.

So, what I have is person.errors and the JSON output I’m looking for is like what’s produced by the hash:

{"errors":{"last_name":["Last name must not be blank"]}}

What do I have to do to get the DataMapper::Validations::ValidationErrors into the JSON format I want?

Answer 1

So, as I was typing this up, the answer came to me (of course!). I’ve burned several hours trying to figure this out, and I hope this will save others the pain and frustration I’ve experienced.

To get the JSON I’m looking for, I just had to create a hash like this:

{ :errors => person.errors.to_h }.to_json

So, now my Sinatra route looks like this:

get '/person' do
    person = Person.new :first_name => 'Dave'
    if person.save
        person.to_json
    else
        { :errors => person.errors.to_h }.to_json
    end
end

Hope this helps others looking to solve this problem.