ordered in RSpec verifies that mocked messages arrive in a specific sequence, helping catch incorrect call order in tests.

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.

Single-listener event handling can cut setup cost and support dynamically added elements, but it can slow fast events and limits bubbling control.

Common REST API design mistakes keep recurring, and a set of practical best practices helps avoid brittle, confusing interfaces.

Compose a custom RSpec matcher from existing matchers to reduce repeated expectations and keep failure messages informative.

Named placeholders in sprintf and format make long strings with several values easier to build, especially URLs and API endpoints with fixed parameters.

simple_format does not escape unsafe HTML and sanitizes generated paragraphs instead. Escape user input first or customize sanitize_options in Rails 7.1.

Accessing Rails path helpers from plain Ruby classes can be awkward, and including all route helpers risks method clashes. Delegating url_helpers exposes only the needed path methods.

Exact fractions in Ruby prevent rounding drift in unit conversions and tests, but mixing Rational with BigDecimal or floats can still reintroduce inaccuracies.

Browser default styles affect pages with no CSS and differ between Chrome, Firefox, and Safari, making layout comparisons and reset strategies inconsistent.

Keep Rails apps protected with a minimally restrictive Content Security Policy that blocks injected scripts while preserving compatibility with nonces and strict-dynamic.

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

Rails ActiveRecord::Relation#merge can silently drop conditions on the same column; a subquery keeps combined scopes unambiguous.

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.

Rails can keep subfolders for organization without turning them into namespaces; collapse lets nested files define top-level constants instead of modules.

CarrierWave 3.x changes upload allowlists, inactive versions, version recreation, and processing paths, breaking older uploader logic and custom transcoders.

Sensitive file attachments can be restricted to authorized users, or served from hashed public paths and expiring URLs for safer access without exposing direct storage locations.

\s misses non-breaking spaces in UTF-8 text; [[:space:]] and \p{Zs} match space separators more reliably, with encoding compatibility caveats.

Rails console assignments can print TypeError (nil can't be coerced into Integer) even when they succeed. The bug affects IRB with --nomultiline and is fixed in reline 0.2.0.

eager_load and includes can explode 1-n association queries into huge cross products, wasting memory. preload uses separate queries and avoids the row multiplication.

extend places module methods into a singleton class’s ancestor chain, which affects class methods, method lookup, and super behavior.

CarrierWave hides MiniMagick processing failures behind a generic error; enabling MiniMagick.logger.level = Logger::DEBUG reveals the underlying ImageMagick commands and helps trace the cause.