Active Record Migrations - Ruby on Rails Guides - 0 views
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張 旭 on 12 Apr 16跟 belongs_to 與 has_many 設定對應的 Migrattion
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add_column and remove_column
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allowing your schema and changes to be database independent.
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each migration as being a new 'version' of the database
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each migration modifies it to add or remove tables, columns, or entries
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Active Record will also update your db/schema.rb file to match the up-to-date structure of your database.
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A primary key column called id will also be added implicitly, as it's the default primary key for all Active Record models
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roll this migration back, it will remove the table
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timestamps macro adds two columns, created_at and updated_at
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On databases that support transactions with statements that change the schema, migrations are wrapped in a transaction
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reversible
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use up and down instead of change
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Migrations are stored as files in the db/migrate directory, one for each migration class.
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a UTC timestamp identifying
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Rails uses this timestamp to determine which migration should be run and in what order
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"AddXXXToYYY" or "RemoveXXXFromYYY"
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use a Ruby DSL
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column type as references
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part_number:string:index
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a migration to remove a column
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"CreateXXX"
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change_column_null
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AddUserRefToProducts
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:references
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produce join tables if JoinTable is part of the name
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CreateJoinTable
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The model and scaffold generators will create migrations appropriate for adding a new model.
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enclosed by curly braces and follow the field type
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create_table
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By default, create_table will create a primary key called id
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add an index on the new column
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when using MySQL, the default is ENGINE=InnoDB
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create_join_table creates an HABTM (has and belongs to many) join table
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To customize the name of the table, provide a :table_name option:
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create_join_table also accepts a block
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change_table, used for changing existing tables
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remove
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rename
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add_column
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change_column
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remove_column
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change_column_default
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place an SQL fragment in the :options option.
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limit
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precision
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scale
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polymorphic
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default
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index
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add_foreign_key
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Active Record only supports single column foreign keys.
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use the old style of migration using up and down methods instead of the change method.
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.connection.execute
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change_table is also reversible, as long as the block does not call change, change_default or remove.
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remove_column is reversible if you supply the column type as the third argument
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Complex migrations may require processing that Active Record doesn't know how to reverse
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reversible
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Using reversible will ensure that the instructions are executed in the right order too.
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add_column add_foreign_key add_index add_reference add_timestamps change_column_default (must supply a :from and :to option) change_column_null create_join_table create_table disable_extension drop_join_table drop_table (must supply a block) enable_extension remove_column (must supply a type) remove_foreign_key (must supply a second table) remove_index remove_reference remove_timestamps rename_column rename_index rename_table
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:column_options option
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have the option :null set to false by default
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By default, the name of the join table comes from the union of the first two arguments provided to create_join_table
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in alphabetical order
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change_column command is irreversible.
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If the column names can not be derived from the table names, you can use the :column and :primary_key options.
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figure out the column name
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foreign key for a specific column
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foreign key by name
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Active Record knows how to reverse the migration automatically
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should use reversible or write the up and down methods instead of using the change method
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If your migration is irreversible, you should raise ActiveRecord::IrreversibleMigration from your down method.
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DontUseConstraintForZipcodeValidationMigration
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rails db:migrate
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the db:migrate task also invokes the db:schema:dump task, which will update your db/schema.rb file to match the structure of your database.
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specify a target version
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all migrations up to and including 20080906120000
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run the down method on all the migrations down to, but not including, 20080906120000
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rails db:rollback
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db:migrate:redo task is a shortcut for doing a rollback and then migrating back up again
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STEP parameter
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db:setup task will create the database, load the schema and initialize it with the seed data
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db:reset task will drop the database and set it up again. This is functionally equivalent to rails db:drop db:setup.
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run a specific migration up or down, the db:migrate:up and db:migrate:down
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the RAILS_ENV environment variable
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db:migrate VERBOSE=false will suppress all output.
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If you have already run the migration, then you cannot just edit the migration and run the migration again: Rails thinks it has already run the migration and so will do nothing when you run rails db:migrate.
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must rollback the migration (for example with bin/rails db:rollback), edit your migration and then run rails db:migrate to run the corrected version.
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editing existing migrations is not a good idea.
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should write a new migration that performs the changes you require
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revert method can be helpful when writing a new migration to undo previous migrations in whole or in part
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require_relative
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revert
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They are not designed to be edited, they just represent the current state of the database.
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Schema Files for
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Schema files are also useful if you want a quick look at what attributes an Active Record object has
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annotate_models gem automatically adds and updates comments at the top of each model summarizing the schema if you desire that functionality.
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database-independent
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multiple databases
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db/schema.rb cannot express database specific items such as triggers, stored procedures or check constraints
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you can execute custom SQL statements, the schema dumper cannot reconstitute those statements from the database
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db:structure:dump
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資料庫種類不相依的 schema 付出的代價就是有些特殊的資料庫特性無法描述出來，例如 trigger；如果有在 migration 寫 SQL 的，簡單說 schema dumper 這邊就要設定成 :sql 而不是預設的 :ruby
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set in config/application.rb by the config.active_record.schema_format setting, which may be either :sql or :ruby.
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check them into source control.
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db/schema.rb contains the current version number of the database
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Validations such as validates :foreign_key, uniqueness: true are one way in which models can enforce data integrity
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The :dependent option on associations allows models to automatically destroy child objects when the parent is destroyed.
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Migrations can also be used to add or modify data
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Initial
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To add initial data after a database is created, Rails has a built-in 'seeds' feature that makes the process quick and easy.
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db/seeds.rb
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rails db:seed
