What is the most efficient way to unit test while mocking out the ORM?

I'm currently in the process of writing unit tests for a small website running Ruby on Sinatra. We're using MiniTest for our unit tests and are starting to run into some issues with maintaining test integrity with our data.

We currently run a test database that is cleared and reset between each 'set' of tests. As our tests grow, this process is beginning to take much longer than we anticipated. In addition, there's concern that we should be keeping each individual test separate instead of using the same database instance for each set.

I've started looking into mocks, but I'm not sure if I'm going down the right path here. I'm having trouble on finding much documentation on this.

Currently, we have something like this:

---

To be tested:

def run(user_id)
    if user_id < 100
        return User.get_by_id(user_id)
    end

    false
end

The test:

describe 'run' do
    before do
        reset_environment
    end

    it 'should return false if the user_id is >= 100' do
        run(100).must_equal false
    end

    it 'should return User object if user_id is < 100' do
        run(99).must_be_instance_of User
    end
end

---

As I said, the tests were starting to take forever to run, and it seemed incredible inefficient. So I'm trying to rewrite things by mocking out the user (in this simplified example). Is this correct?

---

To be tested:

def run(user_id, User: User)
    if user_id < 100
        return User.get_by_id(user_id)
    end

    false
end

The test:

describe 'run' do
    user_mock = MiniTest::Mock.new
    user_mock.expect(:get_by_id, true, [Integer])

    subject (:run_with_mock) { |user_id| run(user_id, user_mock) }

    it 'should return false if the  user_id is >= 100' do
        run_with_mock(100).must_equal false
    end

    it 'should return User object if the user_id is < 100' do
        run_with_mock(99).must_equal true
    end
end

---

Is this the correct way to be going about things, or am I completely off base? It seems to work, but it seems incredibly counter-intuitive.