schema_modification.rdoc

doc/schema_modification.rdoc
Last Update: 2016-11-22 08:26:57 -0800

Schema modification methods

Here's a brief description of the most common schema modification methods:

create_table

create_table is the most common schema modification method, and it's used for adding new tables to the database. You provide it with the name of the table as a symbol, as well a block:

create_table(:artists) do
  primary_key :id
  String :name
end

Note that if you want a primary key for the table, you need to specify it, Sequel does not create one by default.

Column types

Most method calls inside the create_table block will create columns, since method_missing calls column. Columns are generally created by specifying the column type as the method name, followed by the column name symbol to use, and after that any options that should be used. If the method is a ruby class name that Sequel recognizes, Sequel will transform it into the appropriate type for the given database. So while you specified String, Sequel will actually use varchar or text depending on the underlying database. Here's a list of all ruby classes that Sequel will convert to database types:

create_table(:columns_types) do       # common database type used
  Integer :a0                         # integer
  String :a1                          # varchar(255)
  String :a2, :size=>50               # varchar(50)
  String :a3, :fixed=>true            # char(255)
  String :a4, :fixed=>true, :size=>50 # char(50)
  String :a5, :text=>true             # text
  File :b                             # blob
  Fixnum :c                           # integer
  Bignum :d                           # bigint
  Float :e                            # double precision
  BigDecimal :f                       # numeric
  BigDecimal :f2, :size=>10           # numeric(10)
  BigDecimal :f3, :size=>[10, 2]      # numeric(10, 2)
  Date :g                             # date
  DateTime :h                         # timestamp
  Time :i                             # timestamp
  Time :i2, :only_time=>true          # time
  Numeric :j                          # numeric
  TrueClass :k                        # boolean
  FalseClass :l                       # boolean
end

Note that in addition to the ruby class name, Sequel also pays attention to the column options when determining which database type to use. Also note that for boolean columns, you can use either TrueClass or FalseClass, they are treated the same way (ruby doesn't have a Boolean class).

Also note that this conversion is only done if you use a supported ruby class name. In all other cases, Sequel uses the type specified verbatim:

create_table(:columns_types) do  # database type used
  string :a1                     # string
  datetime :a2                   # datetime
  blob :a3                       # blob
  inet :a4                       # inet
end

In addition to specifying the types as methods, you can use the column method and specify the types as the second argument, either as ruby classes, symbols, or strings:

create_table(:columns_types) do  # database type used
  column :a1, :string            # string
  column :a2, String             # varchar(255)
  column :a3, 'string'           # string
  column :a4, :datetime          # datetime
  column :a5, DateTime           # timestamp
  column :a6, 'timestamp(6)'     # timestamp(6)
end

If you use a ruby class as the type, Sequel will try to guess the appropriate type name for the database you are using. If a symbol or string is used as the type, it is used verbatim as the type name in SQL, with the exception of :Bignum. Using the symbol :Bignum as a type will use the appropriate 64-bit integer type for the database you are using.

Column options

When using the type name as method, the third argument is an options hash, and when using the column method, the fourth argument is the options hash. The following options are supported:

:default

The default value for the column.

:index

Create an index on this column. If given a hash, use the hash as the options for the index.

:null

Mark the column as allowing NULL values (if true), or not allowing NULL values (if false). If unspecified, will default to whatever the database default is.

:primary_key

Mark this column as the primary key. This is used instead of the primary key method if you want a non-autoincrementing primary key.

:primary_key_constraint_name

The name to give the primary key constraint.

:type

Overrides the type given as the method name or a separate argument. Not usually used by column itself, but often by other methods such as primary_key or foreign_key.

:unique

Mark the column as unique, generally has the same effect as creating a unique index on the column.

:unique_constraint_name

The name to give the unique key constraint.

Other methods

In addition to the column method and other methods that create columns, there are other methods that can be used:

primary_key

You've seen this one used already. It's used to create an autoincrementing integer primary key column.

create_table(:a0){primary_key :id}

If you want an autoincrementing 64-bit integer:

create_table(:a0){primary_key :id, :type=>:Bignum}

If you want to create a primary key column that doesn't use an autoincrementing integer, you should not use this method. Instead, you should use the :primary_key option to the column method or type method:

create_table(:a1){Integer :id, :primary_key=>true} # Non autoincrementing integer primary key
create_table(:a2){String :name, :primary_key=>true} # varchar(255) primary key

If you want to create a composite primary key, you should call the primary_key method with an array of column symbols. You can provide a specific name to use for the primary key constraint via the :name option:

create_table(:items) do
  Integer :group_id
  Integer :position
  primary_key [:group_id, :position], :name=>:items_pk
end

If provided with an array, primary_key does not create a column, it just sets up the primary key constraint.

foreign_key

foreign_key is used to create a foreign key column that references a column in another table (or the same table). It takes the column name as the first argument, the table it references as the second argument, and an options hash as its third argument. A simple example is:

create_table(:albums) do
  primary_key :id
  foreign_key :artist_id, :artists
  String :name
end

foreign_key accepts the same options as column. For example, to have a unique foreign key with varchar(16) type:

foreign_key :column_name, :table, :unique=>true, :type=>'varchar(16)'

foreign_key also accepts some specific options:

:deferrable

Makes the foreign key constraint checks deferrable, so they aren't checked until the end of the transaction.

:foreign_key_constraint_name

The name to give the foreign key constraint.

:key

The column in the associated table that this column references. Unnecessary if this column references the primary key of the associated table, at least on most databases.

:on_delete

Specify the behavior of this foreign key column when the row with the primary key it references is deleted, can be :restrict, :cascade, :set_null, or :set_default. You can also use a string, which is used literally.

:on_update

Specify the behavior of this foreign key column when the row with the primary key it references modifies the value of the primary key. Takes the same options as :on_delete.

Like primary_key, if you provide foreign_key with an array of symbols, it will not create a column, but create a foreign key constraint:

create_table(:artists) do
  String :name
  String :location
  primary_key [:name, :location]
end
create_table(:albums) do
  String :artist_name
  String :artist_location
  String :name
  foreign_key [:artist_name, :artist_location], :artists
end

When using an array of symbols, you can also provide a :name option to name the constraint:

create_table(:albums) do
  String :artist_name
  String :artist_location
  String :name
  foreign_key [:artist_name, :artist_location], :artists, :name=>'albums_artist_name_location_fkey'
end

If you want to add a foreign key for a single column with a named constraint, you must use the array form with a single symbol:

create_table(:albums) do
  primary_key :id
  Integer :artist_id
  String :name
  foreign_key [:artist_id], :artists, :name=>'albums_artist_id_fkey'
end

index

index creates indexes on the table. For single columns, calling index is the same as using the :index option when creating the column:

create_table(:a){Integer :id, :index=>true}
# Same as:
create_table(:a) do
  Integer :id
  index :id
end

create_table(:a){Integer :id, :index=>{:unique=>true}}
# Same as:
create_table(:a) do
  Integer :id
  index :id, :unique=>true
end

Similar to the primary_key and foreign_key methods, calling index with an array of symbols will create a multiple column index:

create_table(:albums) do
  primary_key :id
  foreign_key :artist_id, :artists
  Integer :position
  index [:artist_id, :position]
end

The index method also accepts some options:

:name

The name of the index (generated based on the table and column names if not provided).

:type

The type of index to use (only supported by some databases)

:unique

Make the index unique, so duplicate values are not allowed.

:where

Create a partial index (only supported by some databases)

unique

The unique method creates a unique constraint on the table. A unique constraint generally operates identically to a unique index, so the following three create_table blocks are pretty much identical:

create_table(:a){Integer :a, :unique=>true}

create_table(:a) do
  Integer :a
  index :a, :unique=>true
end

create_table(:a) do
  Integer :a
  unique :a
end

Just like index, unique can set up a multiple column unique constraint, where the combination of the columns must be unique:

create_table(:a) do
  Integer :a
  Integer :b
  unique [:a, :b]
end

full_text_index and spatial_index

Both of these create specialized index types supported by some databases. They both take the same options as index.

constraint

constraint creates a named table constraint:

create_table(:artists) do
  primary_key :id
  String :name
  constraint(:name_min_length){char_length(name) > 2}
end

Instead of using a block, you can use arguments that will be handled similarly to Dataset#where:

create_table(:artists) do
  primary_key :id
  String :name
  constraint(:name_length_range, Sequel.function(:char_length, :name)=>3..50)
end

check

check operates just like constraint, except that it doesn't take a name and it creates an unnamed constraint:

create_table(:artists) do
  primary_key :id
  String :name
  check{char_length(name) > 2}
end

It's recommended that you use the constraint method and provide a name for the constraint, as that makes it easier to drop the constraint later if necessary.

create_join_table

create_join_table is a shortcut that you can use to create simple many-to-many join tables:

create_join_table(:artist_id=>:artists, :album_id=>:albums)

which expands to:

create_table(:albums_artists) do
  foreign_key :album_id, :albums, :null=>false
  foreign_key :artist_id, :artists, :null=>false
  primary_key [:album_id, :artist_id]
  index [:artist_id, :album_id]
end

create_table :as=>

To create a table from the result of a SELECT query, instead of passing a block to create_table, provide a dataset to the :as option:

create_table(:older_items, :as=>DB[:items].where{updated_at < Date.today << 6})

alter_table

alter_table is used to alter existing tables, changing their columns, indexes, or constraints. It it used just like create_table, accepting a block which is instance_evaled, and providing its own methods:

add_column

One of the most common methods, add_column is used to add a column to the table. Its API is similar to that of create_table's column method, where the first argument is the column name, the second is the type, and the third is an options hash:

alter_table(:albums) do
  add_column :copies_sold, Integer, :default=>0
end

drop_column

As you may expect, drop_column takes a column name and drops the column. It's often used in the down block of a migration to drop a column added in an up block:

alter_table(:albums) do
  drop_column :copies_sold
end

rename_column

rename_column is used to rename a column. It takes the old column name as the first argument, and the new column name as the second argument:

alter_table(:albums) do
  rename_column :copies_sold, :total_sales
end

add_primary_key

If you forgot to include a primary key on the table, and want to add one later, you can use add_primary_key. A common use of this is to make many_to_many association join tables into real models:

alter_table(:albums_artists) do
  add_primary_key :id
end

Just like create_table's primary_key method, if you provide an array of symbols, Sequel will not add a column, but will add a composite primary key constraint:

alter_table(:albums_artists) do
  add_primary_key [:album_id, :artist_id]
end

If you just want to take an existing single column and make it a primary key, call add_primary_key with an array with a single symbol:

alter_table(:artists) do
  add_primary_key [:id]
end

add_foreign_key

add_foreign_key can be used to add a new foreign key column or constraint to a table. Like add_primary_key, if you provide it with a symbol as the first argument, it creates a new column:

alter_table(:albums) do
  add_foreign_key :artist_id, :artists
end

If you want to add a new foreign key constraint to an existing column, you provide an array with a single element. It's encouraged to provide a name when adding the constraint, via the :name option:

alter_table(:albums) do
  add_foreign_key [:artist_id], :artists, :name=>:albums_artist_id_fkey
end

To set up a multiple column foreign key constraint, use an array with multiple column symbols:

alter_table(:albums) do
  add_foreign_key [:artist_name, :artist_location], :artists, :name=>:albums_artist_name_location_fkey
end

drop_foreign_key

drop_foreign_key is used to drop foreign keys from tables. If you provide a symbol as the first argument, it drops both the foreign key constraint and the column:

alter_table(:albums) do
  drop_foreign_key :artist_id
end

If you want to just drop the foreign key constraint without dropping the column, use an array. It's encouraged to use the :name option to provide the constraint name to drop, though on some databases Sequel may be able to find the name through introspection:

alter_table(:albums) do
  drop_foreign_key [:artist_id], :name=>:albums_artist_id_fkey
end

An array is also used to drop a composite foreign key constraint:

alter_table(:albums) do
  drop_foreign_key [:artist_name, :artist_location], :name=>:albums_artist_name_location_fkey
end

If you do not provide a :name option and Sequel is not able to determine the name to use, it will probably raise a Sequel::Error exception.

add_index

add_index works just like create_table's index method, creating a new index on the table:

alter_table(:albums) do
  add_index :artist_id
end

It accepts the same options as create_table's index method, and you can set up a multiple column index using an array:

alter_table(:albums_artists) do
  add_index [:album_id, :artist_id], :unique=>true
end

drop_index

As you may expect, drop_index drops an existing index:

alter_table(:albums) do
  drop_index :artist_id
end

Just like drop_column, it is often used in the down block of a migration.

To drop an index with a specific name, use the :name option:

alter_table(:albums) do
  drop_index :artist_id, :name=>:artists_id_index
end

add_full_text_index, add_spatial_index

Corresponding to create_table's full_text_index and spatial_index methods, these two methods create new indexes on the table.

add_constraint

This adds a named constraint to the table, similar to create_table's constraint method:

alter_table(:albums) do
  add_constraint(:name_min_length){char_length(name) > 2}
end

There is no method to add an unnamed constraint, but you can pass nil as the first argument of add_constraint to do so. However, it's not recommended to do that as it is difficult to drop such a constraint.

add_unique_constraint

This adds a unique constraint to the table, similar to create_table's unique method. This usually has the same effect as adding a unique index.

alter_table(:albums) do
  add_unique_constraint [:artist_id, :name]
end

drop_constraint

This method drops an existing named constraint:

alter_table(:albums) do
  drop_constraint(:name_min_length)
end

There is no database independent method to drop an unnamed constraint. Generally, the database will give it a name automatically, and you will have to figure out what it is. For that reason, you should not add unnamed constraints that you ever might need to remove.

On some databases, you must specify the type of constraint via a :type option:

alter_table(:albums) do
  drop_constraint(:albums_pk, :type=>:primary_key)
  drop_constraint(:albums_fk, :type=>:foreign_key)
  drop_constraint(:albums_uk, :type=>:unique)
end

set_column_default

This modifies the default value of a column:

alter_table(:albums) do
  set_column_default :copies_sold, 0
end

To remove a default value for a column, use nil as the value:

alter_table(:albums) do
  set_column_default :copies_sold, nil
end

set_column_type

This modifies a column's type. Most databases will attempt to convert existing values in the columns to the new type:

alter_table(:albums) do
  set_column_type :copies_sold, :Bignum
end

You can specify the type as a string or symbol, in which case it is used verbatim, or as a supported ruby class or the :Bignum symbol, in which case it gets converted to an appropriate database type.

set_column_allow_null

This allows you to set the column as allowing NULL values:

alter_table(:albums) do
  set_column_allow_null :artist_id
end

set_column_not_null

This allows you to set the column as not allowing NULL values:

alter_table(:albums) do
  set_column_not_null :artist_id
end

Other Database schema modification methods

Sequel::Database has many schema modification instance methods, most of which are shortcuts to the same methods in alter_table. The following Database instance methods just call alter_table with a block that calls the method with the same name inside the alter_table block with all arguments after the first argument (which is used as the table name):

  • add_column

  • drop_column

  • rename_column

  • add_index

  • drop_index

  • set_column_default

  • set_column_type

For example, the following two method calls do the same thing:

alter_table(:artists){add_column :copies_sold, Integer}
add_column :artists, :copies_sold, Integer

There are some other schema modification methods that have no alter_table counterpart:

drop_table

drop_table takes multiple arguments and treats all arguments as a table name to drop:

drop_table(:albums_artists, :albums, :artists)

Note that when dropping tables, you may need to drop them in a specific order if you are using foreign keys and the database is enforcing referential integrity. In general, you need to drop the tables containing the foreign keys before the tables containing the primary keys they reference.

drop_table?

drop_table? is similar to drop_table, except that it only drops the table if the table already exists. On some databases, it uses IF NOT EXISTS, on others it does a separate query to check for existence.

This should not be used inside migrations, as if the table does not exist, it may mess up the migration.

rename_table

You can rename an existing table using rename_table. Like rename_column, the first argument is the current name, and the second is the new name:

rename_table(:artist, :artists)

create_table!

create_table! drops the table if it exists before attempting to create it, so:

create_table!(:artists) do
  primary_key :id
end

is the same as:

drop_table?(:artists)
create_table(:artists) do
  primary_key :id
end

It should not be used inside migrations, as if the table does not exist, it may mess up the migration.

create_table?

create_table? only creates the table if it does not already exist, so:

create_table?(:artists) do
  primary_key :id
end

is the same as:

unless table_exists?(:artists)
  create_table(:artists) do
    primary_key :id
  end
end

Like create_table!, it should not be used inside migrations.

create_view and create_or_replace_view

These can be used to create views. The difference between them is that create_or_replace_view will unconditionally replace an existing view of the same name, while create_view will probably raise an error. Both methods take the name as the first argument, and either an string or a dataset as the second argument:

create_view(:gold_albums, DB[:albums].where{copies_sold > 500000})
create_or_replace_view(:gold_albums, "SELECT * FROM albums WHERE copies_sold > 500000")

drop_view

drop_view drops existing views. Just like drop_table, it can accept multiple arguments:

drop_view(:gold_albums, :platinum_albums)