Active Record Validations Expert

Write correct, layered validations for Rails 8.1 applications. Pair every model validation with appropriate database constraints. Never rely on model validations alone for data integrity.

Philosophy

1. Validations are UX, constraints are safety — Model validations produce friendly error messages. DB constraints prevent corrupt data. You need both.
2. Validate at the model layer, constrain at the DB layer — validates :email, presence: true AND null: false in the migration. Always.
3. Uniqueness is a race condition — validates :email, uniqueness: true without a unique DB index is a bug. Full stop.
4. Normalize before you validate — Use normalizes (Rails 7.1+) to strip/downcase BEFORE validation runs. Don't validate messy input.
5. Custom validators are for reuse, validate methods are for one-offs — Don't build an EachValidator class for logic used in one model.

Critical Rules — Read These First

Pair validations with DB constraints

# Model
class User < ApplicationRecord
  validates :email, presence: true, uniqueness: true
end

# Migration — without this, anything that bypasses Active Record (bulk imports, raw SQL, other apps) can insert invalid data
class CreateUsers < ActiveRecord::Migration[8.1]
  def change
    create_table :users do |t|
      t.string :email, null: false  # backs presence
      t.timestamps
    end
    add_index :users, :email, unique: true  # backs uniqueness
  end
end

Pairing cheat sheet:

Validation	DB Constraint
presence: true	null: false
uniqueness: true	add_index unique: true
uniqueness: { scope: :tenant_id }	add_index [:tenant_id, :email], unique: true
numericality: { greater_than: 0 }	CHECK constraint (optional but ideal)
inclusion: { in: %w[a b c] }	CHECK constraint or enum type
length: { maximum: 255 }	limit: 255 on column

Uniqueness validation alone is a race condition

# Two concurrent requests can both pass validation and insert duplicates:
validates :slug, uniqueness: true
# Without: add_index :posts, :slug, unique: true

# Handle the DB constraint error in your controller:
def create
  @user = User.new(user_params)
  @user.save!
rescue ActiveRecord::RecordNotUnique
  @user.errors.add(:email, :taken)
  render :new, status: :unprocessable_entity
end

Normalize before validating

class User < ApplicationRecord
  # Rails 7.1+ normalizations — runs before validation
  normalizes :email, with: ->(email) { email.strip.downcase }
  normalizes :name, with: ->(name) { name.strip }

  validates :email, presence: true, uniqueness: true,
    format: { with: URI::MailTo::EMAIL_REGEXP }
end

Without normalization, " Alice@Example.COM " fails uniqueness checks against "alice@example.com" — or worse, creates a duplicate.

Boolean presence is special

# WRONG — false.blank? is true, so this rejects false!
validates :active, presence: true

# RIGHT — validates the value is actually true or false
validates :active, inclusion: { in: [true, false] }

# Also acceptable
validates :active, exclusion: { in: [nil] }

Instructions

Step 1: Choose the Right Validation

Before writing a validation, ask: "Where does this rule belong?"

Rule Type	Where	Example
Data exists	Model + DB null: false	validates :name, presence: true
Data is unique	Model + DB unique index	validates :email, uniqueness: true
Data format	Model only (DB can't regex efficiently)	validates :email, format: { with: /.../ }
Business logic	Model validate method	validate :end_after_start
Referential integrity	DB foreign key	add_foreign_key in migration
Cross-record consistency	DB constraint or service object	CHECK constraint or transaction

Step 2: Use Built-in Validators Correctly

presence

validates :title, presence: true
# Checks: !value.blank? (rejects nil, "", "   ")
# Pair with: null: false in migration

For associations, validate the association, not the foreign key:

# WRONG — only checks the integer column isn't nil
validates :author_id, presence: true

# RIGHT — also verifies the Author record exists
belongs_to :author  # Rails 5+ validates presence by default

belongs_to validates presence by default since Rails 5. To make it optional: belongs_to :author, optional: true.

uniqueness

validates :email, uniqueness: true
validates :name, uniqueness: { scope: :account_id }  # unique per account
validates :slug, uniqueness: { case_sensitive: false }
validates :email, uniqueness: { conditions: -> { where(deleted_at: nil) } }

Always add the matching unique index in a migration.

format

# Use \A and \z for string boundaries, not ^ and $
validates :username, format: { with: /\A[a-z0-9_]+\z/ }

# ^ and $ match LINE boundaries in Ruby, not string boundaries.
# "valid\nevil" passes /^[a-z]+$/ — this is a security hole that enables injection attacks.

numericality

validates :price, numericality: { greater_than: 0 }
validates :quantity, numericality: { only_integer: true, greater_than_or_equal_to: 0 }
validates :discount, numericality: { in: 0..100 }

Gotcha: numericality rejects nil by default. If the field is optional, add allow_nil: true.

Gotcha: Float precision — numericality converts to Float then BigDecimal. For money, use decimal columns with explicit precision/scale and validate as integer cents, or use a money gem.

length

validates :name, length: { minimum: 2, maximum: 100 }
validates :bio, length: { maximum: 500 }
validates :pin, length: { is: 4 }
validates :password, length: { in: 8..128 }

inclusion / exclusion

validates :role, inclusion: { in: %w[admin editor viewer] }
validates :subdomain, exclusion: { in: %w[www admin api] }

# Dynamic:
validates :size, inclusion: { in: ->(record) { record.available_sizes } }

comparison

validates :end_date, comparison: { greater_than: :start_date }
validates :retry_count, comparison: { less_than_or_equal_to: 10 }

confirmation

validates :email, confirmation: true
validates :email_confirmation, presence: true  # confirmation field itself must be present

# Only validate confirmation when email changes:
validates :email, confirmation: true
validates :email_confirmation, presence: true, if: :email_changed?

acceptance

validates :terms, acceptance: true  # virtual attribute, no DB column needed
validates :eula, acceptance: { accept: ["TRUE", "accepted"] }

absence

validates :supplementary_address, absence: true, unless: :has_primary_address?

validates_associated

has_many :line_items
validates_associated :line_items  # calls valid? on each line_item

# Don't put validates_associated on BOTH sides of an association — it creates
# an infinite loop (parent validates child, child validates parent, etc.).
# Parent validates children; children rely on belongs_to for the reverse.

Step 3: Conditional Validations

Use :if / :unless to scope when validations run.

# Symbol — cleanest for named methods
validates :card_number, presence: true, if: :paid_with_card?

# Lambda — fine for one-liners
validates :company_name, presence: true, if: -> { account_type == "business" }

# Group related conditionals with with_options
with_options if: :is_admin? do
  validates :password, length: { minimum: 10 }
  validates :email, presence: true
end

# Combine conditions (ALL :if must be true, NO :unless can be true)
validates :parking_spot, presence: true,
  if: [:has_car?, :works_onsite?],
  unless: -> { remote_employee? }

Prefer named methods over complex lambdas — a method name communicates intent (paid_with_card?) better than inline logic, and it's easier to test independently.

on: context

validates :email, uniqueness: true, on: :create  # skip on update
validates :age, numericality: true, on: :update

# Custom contexts for multi-step forms:
validates :address, presence: true, on: :checkout
# Trigger: user.valid?(:checkout) or user.save(context: :checkout)

Step 4: Custom Validations

validate method (one-off, single model)

class Event < ApplicationRecord
  validate :end_after_start
  validate :not_in_past, on: :create

  private

  def end_after_start
    return if end_date.blank? || start_date.blank?
    if end_date <= start_date
      errors.add(:end_date, "must be after start date")
    end
  end

  def not_in_past
    return if start_date.blank?
    if start_date < Date.current
      errors.add(:start_date, "can't be in the past")
    end
  end
end

Guard against nil with return if field.blank? — custom validations receive nil when the field is empty, which causes NoMethodError. Let presence handle the "is it present?" check separately.

EachValidator (reusable across models)

# app/validators/email_validator.rb
class EmailValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless URI::MailTo::EMAIL_REGEXP.match?(value)
      record.errors.add(attribute, options[:message] || "is not a valid email")
    end
  end
end

# Usage — name becomes the validation key (email:)
class User < ApplicationRecord
  validates :email, email: true
  validates :backup_email, email: true, allow_blank: true
end

Naming convention: XxxValidator → validates :attr, xxx: true. The class name minus Validator, lowercased/underscored.

Validator class (validates_with, complex cross-field)

# app/validators/address_validator.rb
class AddressValidator < ActiveModel::Validator
  def validate(record)
    %i[street city zip].each do |field|
      if record.send(field).blank?
        record.errors.add(field, "is required for a complete address")
      end
    end
  end
end

class Order < ApplicationRecord
  validates_with AddressValidator, if: :shipping_required?
end

Note: validates_with validators are initialized once for the app lifecycle. Storing instance state causes data to leak between validations of different records.

Step 5: Error Handling

Reading errors

record.valid?                          # triggers validations, returns bool
record.errors.full_messages            # ["Name can't be blank", ...]
record.errors[:name]                   # ["can't be blank", "is too short..."]
record.errors.where(:name, :too_short) # [ActiveModel::Error objects]
record.errors.added?(:name, :blank)    # true/false — check specific error type
record.errors.of_kind?(:name, :blank)  # same as added? but doesn't check options

Adding errors manually

errors.add(:email, :invalid)                            # uses i18n default
errors.add(:email, :taken, value: email)                # interpolates %{value}
errors.add(:base, "Something is wrong with this record") # record-level error

Custom error types for programmatic handling

errors.add(:discount, :exceeds_maximum, count: 50)
# In en.yml:
# activerecord.errors.models.order.attributes.discount.exceeds_maximum: "cannot exceed %{count}%"

Displaying in views

<% if @record.errors.any? %>
  <div id="error_explanation">
    <h2><%= pluralize(@record.errors.count, "error") %> prohibited saving:</h2>
    <ul>
      <% @record.errors.each do |error| %>
        <li><%= error.full_message %></li>
      <% end %>
    </ul>
  </div>
<% end %>

Step 6: Strict Validations

Use for programmer errors, not user input errors.

validates :token, presence: { strict: true }
# Raises ActiveModel::StrictValidationFailed instead of adding to errors

validates :token, presence: { strict: TokenMissingError }
# Raises your custom exception

Use strict validations for: internal invariants, system-generated fields, things that should never fail if code is correct. Don't use for: user-facing form fields.

Step 7: Normalizations (Rails 7.1+)

class User < ApplicationRecord
  normalizes :email, with: ->(email) { email.strip.downcase }
  normalizes :phone, with: ->(phone) { phone.gsub(/\D/, "") }
  normalizes :name, with: ->(name) { name.squish }  # collapse whitespace

  # apply: false — skip normalization on nil (default normalizes nil too)
  normalizes :nickname, with: ->(n) { n.strip }, apply_to_nil: false
end

# Normalizations run:
# - On assignment: user.email = " FOO@BAR.COM " → "foo@bar.com"
# - Before validation
# - On finder methods: User.find_by(email: " FOO@BAR.COM ") normalizes the query value

Normalize before you validate. Without this, " Alice@Example.COM " and "alice@example.com" are treated as different values — leading to duplicates, failed lookups, and confusing error messages.

Methods That Skip Validations

These methods write to DB without running validations. Know them:

update_all, update_column, update_columns, insert, insert!, insert_all, insert_all!, upsert, upsert_all, touch, touch_all, toggle!, increment!, decrement!, update_attribute, save(validate: false)

If you use these, your DB constraints are your only safety net. This is why DB constraints matter.

Common Mistakes Agents Make

1. Adding validates :foo, presence: true without null: false in migration — Data will be inconsistent if anything bypasses Active Record.

2. Using validates :email, uniqueness: true without a unique index — Race condition. Two concurrent requests can both pass validation and insert duplicates.

3. Validating boolean presence — validates :active, presence: true rejects false. Use inclusion: { in: [true, false] }.

4. Using ^ and $ in format regexes — Use \A and \z. Line anchors allow injection via newlines.

5. Not guarding nil in custom validate methods — If presence is optional, your custom method gets nil and raises NoMethodError.

6. Putting validates_associated on both sides — Infinite loop. Only parent validates children.

7. Forgetting that numericality rejects nil — Add allow_nil: true if the field is optional.

8. Not normalizing before uniqueness checks — "foo@bar.com" and "Foo@Bar.com" are different without normalization/case_sensitive: false.

9. Validating foreign key column instead of association — validates :author_id, presence: true doesn't verify the author exists. belongs_to :author does.

10. Writing EachValidator for single-model logic — Overkill. Use a validate method unless you need it in 2+ models.

Quick Reference

Validation triggers (run validations)

create, create!, save, save!, update, update!, valid?, invalid?

Options available on all validators

Option	Purpose
:message	Custom error message (String or Proc)
:on	Context — :create, :update, or custom symbol
:if / :unless	Conditional — symbol, proc, or array
:allow_nil	Skip if value is nil
:allow_blank	Skip if value is blank?
:strict	Raise exception instead of adding error

Message interpolation

validates :name, length: { minimum: 3, message: "must be at least %{count} characters" }
# Available: %{value}, %{attribute}, %{model}, %{count}

validates :email, uniqueness: {
  message: ->(object, data) { "#{data[:value]} is already taken" }
}

See the references/ directory for complete examples and edge cases:

• references/built-in-validators.md — All built-in validators with options, edge cases, and normalizations
• references/custom-validators.md — EachValidator, Validator classes, PORO validators
• references/errors-api.md — Errors API (reading, adding, filtering) and I18n configuration
• references/conditional-validations.md — if/unless, with_options, :on context, multi-step forms
• references/db-constraints.md — DB constraint pairing, uniqueness race conditions, numericality edge cases
• references/testing.md — Testing patterns, performance considerations, miscellaneous recipes