This one is an easy one but still tripped me up. I'm working on a project where generators on RSpec stub files are turned off (don't ask; disagreement about the utility of generators between two senior engineers and I lost) and I just got this error:

bundle exec rspec spec/models/metric_type_spec.rb

An error occurred while loading ./spec/models/metric_type_spec.rb.
  RSpec.describe MetricType, type: :model do
    describe "#val_col" do
      it "should return int_val for word_count" do
        h = FactoryBot.create(:metric_type_word_count)
        expect(h.val_col).to eq :int_val

      it "should return float_val for weight" do
        h = FactoryBot.create(:metric_type_weight)
        expect(h.val_col).to eq :float_val

  uninitialized constant MetricType
# ./spec/models/metric_type_spec.rb:1:in `<top (required)>'
No examples found.

And the error turned out to be that this line was missing:

require 'rails_helper'

That line is normally inserted by the generator but I built this spec file from scratch and just missed it. I've stared at that line a thousand, thousand times over the years but I suspect I never realized that what it did was tell RSpec something like this:

"Read the Rails directory structure and autoload all classes in the directories below /app". Doh!