Replace instance method with rspec-mock

Question

The RSpec documentation shows how to mock a class method. How can I replace an instance method instead. Here is a code example using rspec-mocks to make this more concrete:

require 'rspec'

class Foo
  def foo
    "foo"
  end

  def foobar
    foo + "bar"
  end
end

RSpec.describe "Foo" do
  it "mocks foo" do
    allow(Foo).to receive(:foo).and_return("bar")
    foo = Foo.new
    expect(foo.foobar).to eq("barbar")
  end
end

This gives "Foo does not implement: foo". Granted, this is contrived for simplification. In the actual class I'm testing, foo makes a HTTP request, so I want to mock it while writing a spec for foobar. I'd like to do this with rspec-mocks, but if another gem can help here, I'm open to that as well.

spickermann · Accepted Answer · 2022-03-02 23:23:55Z

1

Just stub the method on the instance instead of the class:

RSpec.describe "Foo" do
  it "mocks foo" do
    foo = Foo.new
    allow(foo).to receive(:foo).and_return("bar")
    expect(foo.foobar).to eq("barbar")
  end
end

answered Mar 2, 2022 at 23:23

spickermann

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1 Comment

Code-Apprentice Over a year ago

Thanks for the answer. I had tried this in my actual project and it didn't work. Now it dawned on me that it's because I'm calling the equivalent of foo in my example here from initialize. So maybe I need to rethink the design of my class.

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