Agent Skills: Rails Auto-Assigned Field Validation

Rails Auto-Assigned Field Validation

Problem

Adding validates :number, presence: true to a model that auto-assigns number in a before_create callback causes all creates to fail with a validation error ("number can't be blank"), even though the number is being set.

Context / Trigger Conditions

• Model has a before_create that assigns a field (e.g., sequence number, slug, token)
• Adding a presence: true validation on that field breaks all creates
• Error: "FieldName can't be blank" on new records
• Existing records are fine; only creates fail
• Common with auto-incrementing record numbers: self.number = (Model.maximum(:number) || 0) + 1

Root Cause

Rails callback execution order:

1. before_validation
2. **validates** ← runs HERE
3. before_save
4. before_create   ← field is assigned HERE (too late!)
5. INSERT
6. after_create
7. after_save

When before_create assigns the field, validations have already run — the field is still blank when validated.

Solution

Change before_create to before_validation on: :create:

# ❌ BROKEN — validation runs before the field is assigned
validates :number, :entry_date, presence: true
before_create :assign_number

# ✅ CORRECT — field is assigned before validation
validates :number, :entry_date, presence: true
before_validation :assign_number, on: :create

private

def assign_number
  self.number = (self.class.maximum(:number) || 0) + 1
end

The on: :create guard ensures the callback only runs on create, not on every valid? call (which could re-assign the number on updates).

Verification

# Should now pass
record = Model.new(required_attrs)
record.valid?   # => true (number is now set before validation)
record.save     # => true
record.number   # => 1 (or next sequence)

Example (this codebase)

app/models/corrective_maintenance.rb and preventive_maintenance.rb both auto-assign number with MAX(number) + 1. When validates :number, presence: true was added, all creates broke until the callback was changed:

# app/models/corrective_maintenance.rb
validates :entry_date, :entry_km, :number, presence: true
before_validation :assign_number, on: :create   # was: before_create

private

def assign_number
  self.number = (CorrectiveMaintenance.maximum(:number) || 0) + 1
end

Notes

• This applies to any auto-generated field: slugs, tokens, UUIDs (when generated in Ruby), serial numbers, etc.
• The on: :create is important — without it, assign_number fires on every save, which would reassign the number on updates and break uniqueness
• ActiveRecord unique index (UNIQUE constraint) still provides DB-level protection; the presence validation is a Ruby-layer guard for cleaner error messages
• Race condition mitigation: even with before_validation, MAX()+1 is still raceable; pair with rescue ActiveRecord::RecordNotUnique retry logic in mutations/controllers