Question

Using RSpec, is there a way to compare two hashes where I don’t know what some values will be? I’ve tried using instance_of, but it doesn’t seem to work.

In my test, I am creating a new device object with a POST request and want to make sure I get the right JSON response back. It’s creating a UUID for the device (I’m not using a relational db), but I obviously won’t know what that value will be and don’t really care.

post "/projects/1/devices.json", name: 'New Device'

expected = {'name' => 'New Device', 'uuid' => instance_of(String), 'type' => 'Device'}

JSON.parse(body).should == expected

And I get the following error:

 1) DevicesController API adds a new device to a project
     Failure/Error: JSON.parse(body).should == expected
       expected: {"name"=>"New Device", "uuid"=>#<RSpec::Mocks::ArgumentMatchers::InstanceOf:0x5a27147d @klass=String>, "type"=>"Device"}
            got: {"name"=>"New Device", "uuid"=>"ef773465-7cec-48fd-b2a7-f1da10d1595a", "type"=>"Device"} (using ==)
       Diff:
       @@ -1,4 +1,4 @@
        "name" => "New Device",
        "type" => "Device",
       -"uuid" => #<RSpec::Mocks::ArgumentMatchers::InstanceOf:0x5a27147d @klass=String>
       +"uuid" => "ef773465-7cec-48fd-b2a7-f1da10d1595a"
     # ./spec/api/devices_spec.rb:37:in `(root)'

Answer 1

instance_of is for argument matching:

something.should_receive(:foo).with(instance_of(String))

Instead of using the == operator you could use include. For example:

JSON.parse(body).should include('uuid', 'name' => 'New Device', 'type' => 'Device')

which says that the key uuid must be present with any value and name and type should be present with the specified value. It will also allow other keys in the hash.

I don’t know of a way to achieve exactly what you’re asking for with the built-in matchers.