Factory Girl in MiniTest

I am piggybacking on my earlier question where I have a factory creating a parent and child. I am running my unit tests and have a simple one not passing.

test "invoice can save" do
  invoice = build(:invoice)
  assert invoice.save, "Error message: #{invoice.errors.full_messages}"
end

Returns the following error.

Error message: ["Trips can't be blank"]

But the following code works.

test "invoice can save" do
  invoice = create(:invoice)
  assert invoice.save, "Error message: #{invoice.errors.full_messages}"
end

My understanding is that build should be holding it in memory until save is called. Isn't the first a better test to see if it is successfully saving to the database?