create_with and where can bypass Rails 3 mass-assignment protection, allowing protected attributes to be set from untrusted input.

Set application-wide defaults for URL generation in Rails instead of duplicating them in multiple places as issues arise.

Ruby and Rails deprecations can hide runtime problems and break future upgrades. Typical fixes involve updating gems, replacing obsolete API calls, or adding compatibility shims.

Rails verbose query logs can point every SQL query to lib/active_record/log_subscriber.rb when backtrace silencers are disabled, making sources unhelpful.

Updating Cucumber on Rails 3+ requires refreshing cucumber, capybara, and cucumber-rails, then restoring any custom support setup after installation.

Rails migration column sizes depend on type: :limit controls bytes or characters, and certain values map to MySQL tinyint, smallint, mediumint, bigint, text, mediumtext, or longtext.

Cross-subdomain session cookies in Rails 3 need a shared cookie domain; local development can fail with CSRF token errors when browsers reject the cookie.

Rails FlashHash comparisons ignore the internal used-entry state, so two flashes can look equal even when their next-request behavior differs.

Rails can hand file delivery to Apache with X-Sendfile, reducing application-server load and preventing zero-byte downloads when local files are sent.

Built-in support for Rails and Bootstrap error classes reduces step overrides, while configurable error classes, XPath selectors, and optional negation improve flexibility.

Puma concurrency in Rails depends on workers and threads; self-calls during a request can deadlock in development or tests unless multiple workers are enabled.

Set the Rails environment explicitly when running server-side scripts to avoid using the wrong database or configuration. Rails 2 uses script/runner; Rails 3 uses rails runner.

Rails localization with fast_gettext enables multi-language apps using .po files, _() and n_() for translated text and plural forms.

HTML emails often break when mail clients ignore external stylesheets. premailer-rails inlines CSS for Rails mailers, and inline image attachments improve compatibility.

Version mismatches between Mocha and Rails 3.2 can trigger an already initialized constant warning; updating Mocha and using a matching Rails version avoids it.

Rails gems that must support several framework releases need separate test apps per version. Shared code and specs reduce duplication while keeping version-specific coverage.

Rails 2 and some Rails 3.x versions let chained scopes overwrite hash conditions on the same attribute, creating authorization leaks. chain_safely avoids that overwrite.

Inherited models and concerns can hide which checks run on a Rails model; validators and validators_on reveal them at runtime.

Ruby charting library for Rails that generates PNG graphs with themes, multiple series, and automatic sizing for large datasets.

Render Rails views outside a request for PDFs, email attachments, or background reports using ApplicationController.render or render_to_string.

Rails can nest flat JSON request bodies under a root key so strong parameters work like form submissions and stray keys such as format stop interfering.

Rails constant loading can fail silently in development, especially with threaded code. Logging ActiveSupport::Dependencies helps trace autoloader behavior and diagnose hangs.

Reusable Ruby code can need different behavior across Rails 2 and Rails 3+. Version checks let code branch cleanly for compatibility.