Avoid recreating the subject in RSpec - ruby-on-rails

Question

Say I have the following test
describe "bob" do
subject {
response = get "/expensive_lookup"
JSON.parse(response.body)
}
its(["transaction_id"]) { should == 1 }
its(["order_id"]) { should == 33 }
end
Then for each its() {} the subject will be reevaluated, which in my case it is a very slow lookup.
I could bundle all my tests together in one like
describe "bob" do
subject(res) {
response = get "/expensive_lookup"
JSON.parse(response.body)
}
it "returns the right stuff" do
res["transaction_id"]).should == 1
res["order_id"].should == 33
end
end
But this makes it less obvious which line of the test has failed if there is a failure.
Is there a way I can stop the subject from being reevaluated for each it block?

Answer 1

You can put that in to a before(:all) block. I don't know if that syntax has changed in a new rspec version, but regardless, your test would become this:
before(:all) do
response = get "/expensive_lookup"
#res = JSON.parse(response.body)
end
it "returns the right transaction ID" do
#res["transaction_id"].should == 1
end
# etc
The pro is that the code in the before-all block gets run just once for your spec. The con is that, as you can see, you can't take advantage of the subject; you need to write each more explicitly. Another gotcha is that any data saved to the test database is not part of the transaction and will not be rolled back.

Answer 2

There are two possible source of issues
Network request is slow/prone to fail
You should really mock all you network requests, slow or not.
The gem VCR is excellent. It makes it trivial to run your request once and persist the result for subsequent testing.
Building the immutable subject is slow
If you have multiple it blocks, the subject will be rebuild every time. Assuming you don't modify the subject, you can build it once.
You can use before(:all):
before(:all) { #cache = very_long_computation.freeze }
subject { #cache }
Note: that I call freeze to avoid modifying it by mistake, but of course that's not a deep freeze so you still need to mind what you are doing. If you are mutating your subject, your tests are no longer independent and shouldn't share the subject.