if you wanted to keep it with the 'change' thing.

```ruby
expect{
   post :create, token: valid_token, tag_name: tag.name, value: 'Hello World'
}.to change(MappedValue, :count).by(1)
```

My recommended way:
```ruby
post :create, token: valid_token, tag_name: tag.name, value: 'Hello World'
new_mapped_value = MappedValue.last.name.s
new_mapped_value.name.should eq 'Hello World'
new_mapped_value.tag_name.should eq tag.name
```