class Parent
  embeds_many :children
  accepts_nested_attributes_for :children
end

class Child
  field first_name
  field last_name

  embedded_in :parent
end

describe 'update' do  
  let(:parent) { Parent.create }
  let(:child) { parent.children.create first_name: 'first', last_name: 'last' }
  subject{parent.update params}

  context "params have 'children_attributes' key " do 
    let(:params) { children_attributes: [id: child.id, first_name: 'new']}
    it {expect{subject}.to change{parent.reload.children.first.first_name}.to('new')} # => pass
    it {expect{subject}.not_to change{parent.reload.children.first.last_name}} # => pass
  end 

  context "params have 'children_attributes' key "do 
    let(:params) { children: [id: child.id, first_name: 'new']}
    it {expect{subject}.to change{parent.reload.children.first.first_name}.to('new')} # => pass
    it {expect{subject}.not_to change{parent.reload.children.first.last_name}} # => fail. last_name is nil
  end 

  context  "children is a hash" do 
    let(:params) { children: {"0" => {id: child.id, first_name: 'new'}}}
    it {expect{subject}.to change{parent.reload.children.first.first_name}.to('new')} # => pass
    it {expect{subject}.not_to change{parent.reload.children.first.last_name}} # => pass
  end 
end