activerecord-patterns

ActiveRecord Patterns

Senior Rails ActiveRecord conventions, organized as before/after pattern pairs. AI agents routinely violate these — they reach for callbacks where a job belongs, use where.first when find_by exists, default-scope themselves into corners, and lose database integrity through naïve polymorphic associations. This skill makes the agent write models the way the Rails core team writes them.

The opinion (DHH-leaning default)

Fat models are fine. Put domain logic in the model. Use ActiveSupport::Concern to split fat models into focused mixins. Reach for service objects only when (a) orchestrating multiple models in a single workflow, (b) calling external APIs, or (c) the workflow lives outside an obvious model's domain.

DHH on this:

"I far prefer current_account.posts.visible_to(current_user) to involving a third query object."
— Put Chubby Models on a Diet with Concerns

The counter-position: small-object-per-responsibility (the Sandi Metz / 7±2 doctrine) keeps churn cheaper in very large codebases. We acknowledge that and recommend it only at team-size-large (20+) — see service-objects-vs-fat-models for the full breakdown.

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Core patterns

Pattern 1: find vs find_by vs where.first

The wrong choice silently hides bugs.

Before (AI-typical, semantically wrong):

# Returns nil if no slug match, then NoMethodError on `.title` two screens later.
post = Post.where(slug: params[:slug]).first
post.title

# Or this version that hides ordering bugs:
post = Post.where(author_id: current_user.id).first
# No order specified — "first" is whatever the DB happened to return.

After (Rails-conventional):

# `find_by` is the single-record finder. Returns nil if missing — handle it.
post = Post.find_by(slug: params[:slug])
return head :not_found unless post

# Use the bang form when "missing" must crash:
post = Post.find_by!(slug: params[:slug])

# Use `find(id)` for primary-key lookups — it always raises ActiveRecord::RecordNotFound.
post = Post.find(params[:id])

# When you legitimately need the first of an ordered set, order explicitly:
latest = Post.where(author_id: current_user.id).order(created_at: :desc).first