Gemfile

.env

# Add literally these into env.example, to help newbies that follow you on this app.
# Put the real values into .env
MAILCHIMP_API_KEY=key
MAILCHIMP_LIST_NAME="List name"

candidate.rb

# This is the model I needed to add to the list, add this in there somewhere.

  after_save :subscribe_to_mailchimp
  
  def subscribe_to_mailchimp
    MailchimpInterface.subscribe(
      MailchimpInterface.list_named(
        MailchimpInterface::LIST_NAME), self)
  end

candidate_spec.rb

# # In spec/models/candidate_spec.rb
# This represents the "mock" version of how to test.
# I think it's complicated and hard to read afterwards, but opinions can vary.
# It definitely does require less overall setup.

  context "new candidates", focused: true do
    
    it "should subscribe a new candidate to Mailchimp" do
      expect {
        l = double("list")
        allow(l).to receive(:[]).with("name"){ MailchimpInterface::LIST_NAME }
        allow(MailchimpInterface).to receive(:all_lists){ [l] }
        expect(MailchimpInterface).to receive(:subscribe)
        create(:candidate)
      }.to change(Candidate, :count).by(1)
    end
    
  end 

Gemfile

# Insert this into to your Gemfile

gem 'gibbon'

mailchimp_interface.rb

# Mailchimp interface, should be in app/interfaces/mailchimp_interface.rb

class MailchimpInterface

  LIST_NAME = ENV["MAILCHIMP_LIST_NAME"]
  
  def self.all_lists
    client.lists.list["data"]
  end

  def self.list_named(name)
    all_lists.select{ |l| l["name"] == name}.first
  end

  def self.members(list)
    client.lists.members(id: list["id"])["data"]
  end
  
  # Note that this method has particular requirements
  # of your model layer in terms of field names, etc.
  def self.subscribe(list, user)
    client.lists.subscribe({:id => list["id"], 
      :email => {:email => user.email}, 
      :merge_vars => {:FNAME => user.first_name, :LNAME => user.last_name}, 
      :double_optin => false})
  end
  
  def self.client
    Gibbon::API.new
  end

end