Using RSpec to check if something is an instance of another object

The preferred syntax is:

expect(@object).to be_a Shirt

The older syntax is:

@object.should be_an_instance_of Shirt

Note that there is a very subtle difference between the two. If Shirt were to inherit from Garment then both of these expectations will pass:

expect(@object).to be_a Shirt
expect(@object).to be_a Garment

If you do and @object is a Shirt, then the second expectation will fail:

@object.should be_an_instance_of Shirt
@object.should be_an_instance_of Garment

You mean you want to check if an object is an instance of a class? If so, that's easy, just use class:

@object.class.should == Shirt