Add Test If An Instance Variable Was Assigned as a rails til

rails/test-if-an-instance-variable-was-assigned.md

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+# Test If An Instance Variable Was Assigned
+
+When testing Rails controller actions, you'll often be making assertions about
+the response to your test request. You may also want to assert about the
+instance variables being set, as those are headed for your view layer.
+
+For instance, if an instance variable `@metadata` is supposed to be set in a
+`#show` controller action, you can assert that it is with [RSpec's
+`assigns`](https://relishapp.com/rspec/rspec-rails/docs/controller-specs):
+
+```ruby
+describe "when given valid params" do
+  it "sets the metadata" do
+    get :show, params: valid_params
+
+    expect(assigns(:metadata)).to match(
+      identifier: "abc123",
+      session_id: "fe98f08c-bf2f-4749-9f81-071d9cc7720e",
+    )
+  end
+end
+```
+
+[source](https://stackoverflow.com/questions/2051373/how-to-test-instance-variable-was-instantiated-in-controller-with-rspec)