Bulk inserts into one table can be much faster inside a transaction; Rails 6+ also offers insert_all for a single multi-row INSERT without validations.

Time comparisons can fail in tests because Ruby keeps subsecond fractions even when the printed values look identical.

Date and time interval checks are easy to get wrong because partial, nested, and touching ranges all count as overlaps. A simple inclusive comparison works for Ruby and Rails.

Traits can leak constant values across including classes when constants are assigned inside as_trait; binding them to self keeps each class’s constant separate.

Mixing safe and unsafe HTML strings with Array#join breaks Rails output safety and can trigger incorrect escaping or unsafe rendering.

Virtual attributes can need validation messages shown on a different field, especially with Paperclip attachments and related file-name checks.

Haml supports HTML comments that reach the browser and Ruby comments that stay server-side, with indentation errors causing confusing syntax failures.

Rails environment code loads in a fixed boot order, affecting which settings and constants can override others. Knowing the initialization sequence prevents surprises when adding configuration or app code.

Preloading nested associations on already loaded ActiveRecord objects avoids extra queries when rendering deeply related data and fits legacy Rails models via preload_associations.

git stash apply leaves entries in place, so later stashing can restore the wrong changes; git stash pop applies and removes the top stash entry.