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Feature: Partial test doubles
A _partial test double_ is an extension of a real object in a system that is instrumented with
test-double like behaviour in the context of a test. This technique is very common in Ruby
because we often see class objects acting as global namespaces for methods. For example,
in Rails:
```ruby
person = double("person")
allow(Person).to receive(:find) { person }
```
In this case we're instrumenting Person to return the person object we've defined whenever
it receives the `find` message. We can also set a message expectation so that the example
fails if `find` is not called:
```ruby
person = double("person")
expect(Person).to receive(:find) { person }
```
RSpec replaces the method we're stubbing or mocking with its own test-double like method.
At the end of the example, RSpec verifies any message expectations, and then restores the
original methods.
Note: we recommend enabling the [`verify_partial_doubles`](../verifying-doubles/partial-doubles) config option.
Scenario: Only the specified methods are redefined
Given a file named "partial_double_spec.rb" with:
"""ruby
RSpec.describe "A partial double" do
# Note: stubbing a string like this is a terrible idea.
# This is just for demonstration purposes.
let(:string) { "a string" }
before { allow(string).to receive(:length).and_return(500) }
it "redefines the specified methods" do
expect(string.length).to eq(500)
end
it "does not effect other methods" do
expect(string.reverse).to eq("gnirts a")
end
end
"""
When I run `rspec partial_double_spec.rb`
Then the examples should all pass
Scenario: The original method is restored when the example completes
Given a file named "partial_double_spec.rb" with:
"""ruby
class User
def self.find(id)
:original_return_value
end
end
RSpec.describe "A partial double" do
it "redefines a method" do
allow(User).to receive(:find).and_return(:redefined)
expect(User.find(3)).to eq(:redefined)
end
it "restores the redefined method after the example completes" do
expect(User.find(3)).to eq(:original_return_value)
end
end
"""
When I run `rspec partial_double_spec.rb --order defined`
Then the examples should all pass
|
