Squash Migrations

Squashing migrations is the process of going through individual migrations in db/migrate by date, and "squashing" them into the InitCanvasDb migration, then deleting the original. Canvas' policy is that we squash migrations quarterly, with a one quarter lag. In other words, migrations from Q1 are squashed beginning in Q3. Do one month's worth of migration per commit, to ease review complexity. Plugins in gems/plugins/*/db/migrate might also have migrations that need squashed into a base migration for that plugin, similar to InitCanvasDb. Plugin migrations use a single commit for the entire quarter, since they rarely have migrations.

Rules

• change_table blocks that add new structures can be moved into the corresponding create_table block.
• Individual DDL statements such as add_index, add_reference, etc. should also be moved to the corresponding create_table block, and modified as appropriate if their arguments differ.
• remove_-style statements should result in the removal of the corresponding add_-style structure from InitCanvasDb. Check the model file for any removed columns, and if the column has been ignored there, remove it from the list. Remove the ignored_columns line completely if the list is empty.
• create_table blocks should be moved into InitCanvasDb completely, putting it into its properly alphabetized position (using the non-plural form of the table name, so that for example discussion_topics is placed before discussion_topic_replies)
• set_replication_identity calls are moved into the SetReplicaIdenties migration, in their same alphabetized position.
• Any options (such as algorithm: :concurrently, if_not_exists: true, validate: false, validate_constraint) used to make the original migration idempotent are not necessary in InitCanvasDb, and should be removed.
• Options that are already the default should not be specified:
  • default: nil
  • null: true
  • index: true on t.references calls
  • index: false on non-reference column addition calls
• Keep the statements within create_table blocks organized, with a blank line between each section:
  • Column additions (including t.timestamps) are the first section, preserving their original order they were added
  • Additional constraints are the next section, preserving their original order they were added
  • Additional indexes are the final section, preserving their original order they were added, with the exception that the t.replica_identity_index is first.
• If the migration queues a DataFixup, find the file defining it, and any associated spec file. If the DataFixup is not referenced by any other migration, just delete the spec file, the DataFixup file, and the migration file.
• create_initial_partitions calls can be squashed into the CreateInitialPartitions migration.

Validation Process

Before making any modifications, reset the test database and store a baseline:

RAILS_ENV=test bin/rake db:test:reset
pg_dump -s --restrict-key=MQTD3FxKJiJ5XiNN2cfyqy9ctUI0Tt9i3SWn8wZ7l2dYLJGctear9gqS1IRbdO5 canvas_test > original.sql

After making modifications, reset the test database again, dump it, and confirm no structural changes:

RAILS_ENV=test bin/rake db:test:reset
pg_dump -s --restrict-key=MQTD3FxKJiJ5XiNN2cfyqy9ctUI0Tt9i3SWn8wZ7l2dYLJGctear9gqS1IRbdO5 canvas_test > modified.sql
diff -u original.sql modified.sql

The diff output should be empty. Exceptions are allowed if the order of columns has changed because a squashed column is now earlier in the table than a column added by a migration in gems/plugins/*/db/migrate/*.

Finishing Up

• Alter ValidateMigrationIntegrity by replacing the timestamp in last_squashed_migration_version with the value from the last deleted migration, and increment the version number in the filename.
• Run script/rlint -a to fix any formatting issues.