Mocking a specific instance of an ActiveRecord model that hasn't yet been queried

example_spec.rb

require 'rails_helper'

RSpec.describe 'Mocking specific instance' do
  let!(:student_to_stub) { create_list(:student, 3).first }
  let(:stubbed_name) { junk }
  
  before do
    stub_model(Student, :first_name, { id: student_to_stub.id }, stubbed_name)
  end

  subject { Student.find(student_to_stub.id) }

  it 'stubs matching instances' do
    expect(subject.first_name).to eq stubbed_name
    expect(Student.last.first_name).not_to eq stubbed_name
  end

  def stub_model(klass, method, conditions = {}, return_value = nil)
    allow_any_instance_of(klass).to receive(method).and_wrap_original do |method, *args|
      if conditions.none? || conditions.all? { |k,v| method.receiver.send(k) == v }
        block_given? ? yield(method.receiver, *args) : return_value
      else
        method.call(*args)
      end
    end
  end
end