SQL » Attributes
Relations provide a powerful API for working with their attributes. They can be used to rename
canonical columns, qualify them, compose more complex WHERE conditions, or create SQL functions.
Qualifying and Renaming Attributes
Joining relations introduces a problem of having conflicting attribute names. To solve this you often need to qualify and rename columns.
To qualify all attributes in a relation:
class Users < ROM::Relation[:sql]
schema(infer: true) do
associations do
has_many :tasks
has_many :posts
end
end
def with_tasks
join(:tasks, user_id: :id).qualified
# the same will be done when you use a shortcut:
join(tasks)
end
# produces "SELECT users.id, users.name ..."
end
To rename all attributes in a relation:
class Users < ROM::Relation[:sql]
schema(infer: true) do
associations do
has_many :tasks
has_many :posts
end
end
def with_tasks
prefix(:user).qualified
end
# produces "SELECT users.id AS user_id, users.name AS user_name ..."
end
To rename attributes in block-based DSLs you can use as method:
class Users < ROM::Relation[:sql]
schema(infer: true)
def index
select { [id.as(:user_id), name.as(:user_name) }
end
# produces "SELECT users.id AS user_id, users.name AS user_name ..."
end
Creating functions from attributes
You can use an attribute to create a function. This is useful in cases where you would like to append a function that's created from an attribute coming from another relation. For example:
class Users < ROM::Relation[:sql]
schema(infer: true)
def index
select(:id, :name, tasks[:id].func { int::count(id).as(:task_count) }).
left_join(tasks).
group(:id)
end
# SELECT "users"."id", "users"."name", COUNT("tasks"."id") AS "task_count" FROM "users" LEFT JOIN "tasks" ON ("users"."id" = "tasks"."user_id") GROUP BY "users"."id" ORDER BY "users"."id"
end
Using attributes for restrictions
You can use Attribute API in restriction methods such as #where:
class Users < ROM::Relation[:sql]
schema(infer: true)
def index
where { name.is('Jane') | name.is('Joe') }
end
# SELECT "id", "name" FROM "users" WHERE (("name" = 'Jane') OR ("name" = 'Joe')) ORDER BY "users"."id""
end
Learn more
Check out API documentation: