Phase 1: Expand (v2.1.0 - MINOR)

									ALTER TABLE users ADD COLUMN first_name VARCHAR(100);
									ALTER TABLE users ADD COLUMN last_name VARCHAR(100);
								

Add new columns, dual-write to both, continue reading from old

All phases are MINOR (additive) until final DROP (MAJOR)

Phase 1: Expand - Dual-Write Code

Application keeps both columns synchronized:

• If full_name changes → split and write to first_name / last_name
• If first_name / last_name change → combine to full_name
• All writes update both formats

Phase 1: Expand - Implementation

									class User < ApplicationRecord
										before_save :sync_name_fields

										def sync_name_fields
											# Write to both old and new columns
											if will_save_change_to_full_name?
												parts = full_name.to_s.split(' ', 2)
												self.first_name = parts[0] || ''
												self.last_name = parts[1] || ''
											elsif will_save_change_to_first_name? || will_save_change_to_last_name?
												self.full_name = "#{first_name} #{last_name}".strip
											end
										end
									end
								

Reads prioritize new fields with fallback to old

Phase 2: Backfill (background job)

									UPDATE users
									SET
										first_name = SPLIT_PART(full_name, ' ', 1),
										last_name = COALESCE(NULLIF(SPLIT_PART(full_name, ' ', 2), ''), '')
									WHERE
										full_name IS NOT NULL
										AND (first_name IS NULL OR last_name IS NULL);
								

• Run as background job (not in migration)
• Process in batches | Can take hours/days
• Application handles missing data gracefully

Phase 3: Switch Reads (v2.3.0 - MINOR)

									# Code now READS from first_name/last_name
									# But still WRITES to both (dual-write continues)
									def full_name
									  "#{first_name} #{last_name}".strip
									end
								

Phase 4: Stop Old Writes (v2.4.0 - MINOR)

									class User < ApplicationRecord
										# sync_name_fields callback REMOVED
										# full_name column is now ignored (becomes stale)

										# All writes go exclusively to new columns
										def update_name(first, last)
											update!(first_name: first, last_name: last)
											# full_name is NOT updated anymore
										end
									end
								

• Remove the sync_name_fields callback from Phase 1
• Writes go exclusively to first_name / last_name
• full_name becomes stale — safe to drop in next phase

Phase 5: Contract - Drop Old Column (v3.0.0 - MAJOR)

									ALTER TABLE users DROP COLUMN full_name;
								

Ensure new code works before removing safety net