Scenic adds methods to ActiveRecord::Migration
to create and manage database
views in Rails.
Using Scenic, you can bring the power of SQL views to your Rails application without having to switch your schema format to SQL. Scenic provides a convention for versioning views that keeps your migration history consistent and reversible and avoids having to duplicate SQL strings across migrations. As an added bonus, you define the structure of your view in a SQL file, meaning you get full SQL syntax highlighting in the editor of your choice and can easily test your SQL in the database console during development.
Scenic ships with support for PostgreSQL. The adapter is configurable (see
Scenic::Configuration
) and has a minimal interface (see
Scenic::Adapters::Postgres
) that other gems can provide.
You've got this great idea for a view you'd like to call search_results
. You
can create the migration and the corresponding view definition file with the
following command:
$ rails generate scenic:view search_results
create db/views/search_results_v01.sql
create db/migrate/[TIMESTAMP]_create_search_results.rb
Edit the db/views/search_results_v01.sql
file with the SQL statement that
defines your view. In our example, this might look something like this:
SELECT
statuses.id AS searchable_id,
'Status' AS searchable_type,
comments.body AS term
FROM statuses
JOIN comments ON statuses.id = comments.status_id
UNION
SELECT
statuses.id AS searchable_id,
'Status' AS searchable_type,
statuses.body AS term
FROM statuses
The generated migration will contain a create_view
statement. Run the
migration, and baby, you got a view going. The migration is reversible
and the schema will be dumped into your schema.rb
file.
$ rake db:migrate
Here's where Scenic really shines. Run that same view generator once more:
$ rails generate scenic:view search_results
create db/views/search_results_v02.sql
create db/migrate/[TIMESTAMP]_update_search_results_to_version_2.rb
Scenic detected that we already had an existing search_results
view at version
1, created a copy of that definition as version 2, and created a migration to
update to the version 2 schema. All that's left for you to do is tweak the
schema in the new definition and run the update_view
migration.
The update_view
statement used by default will drop your view then create
a new version of it.
This is not desirable when you have complicated hierarchies of views, especially when some of those views may be materialized and take a long time to recreate.
You can use replace_view
to generate a CREATE OR REPLACE VIEW SQL statement.
See postgresql documentation on how this works: http://www.postgresql.org/docs/current/static/sql-createview.html
To start replacing a view run the generator like for a regular change:
$ rails generate scenic:view search_results
create db/views/search_results_v02.sql
create db/migrate/[TIMESTAMP]_update_search_results_to_version_2.rb
Now, edit the migration. It should look something like:
class UpdateSearchResultsToVersion2 < ActiveRecord::Migration
def change
update_view :search_results, version: 2, revert_to_version: 1
end
end
Update it to use replace view:
class UpdateSearchResultsToVersion2 < ActiveRecord::Migration
def change
replace_view :search_results, version: 2, revert_to_version: 1
end
end
Now you can run the migration like normal.
You bet! Using view-backed models can help promote concepts hidden in your relational data to first-class domain objects and can clean up complex ActiveRecord or ARel queries. As far as ActiveRecord is concerned, a view is no different than a table.
class SearchResult < ActiveRecord::Base
belongs_to :searchable, polymorphic: true
private
# this isn't strictly necessary, but it will prevent
# rails from calling save, which would fail anyway.
def readonly?
true
end
end
Scenic even provides a scenic:model
generator that is a superset of
scenic:view
. It will act identically to the Rails model
generator except
that it will create a Scenic view migration rather than a table migration.
There is no special base class or mixin needed. If desired, any code the model generator adds can be removed without worry.
$ rails generate scenic:model recent_status
invoke active_record
create app/models/recent_status.rb
invoke test_unit
create test/models/recent_status_test.rb
create test/fixtures/recent_statuses.yml
create db/views/recent_statuses_v01.sql
create db/migrate/20151112015036_create_recent_statuses.rb
Materialized views are essentially SQL queries whose results can be cached to a table, indexed, and periodically refreshed when desired. Does Scenic support those? Of course!
The scenic:view
and scenic:model
generators accept a --materialized
option for this purpose. When used with the model generator, your model will
have the following method defined as a convenience to aid in scheduling
refreshes:
def self.refresh
Scenic.database.refresh_materialized_view(table_name, concurrently: false, cascade: false)
end
This will perform a non-concurrent refresh, locking the view for selects until
the refresh is complete. You can avoid locking the view by passing
concurrently: true
but this requires both PostgreSQL 9.4 and your view to have
at least one unique index that covers all rows. You can add or update indexes for
materialized views using table migration methods (e.g. add_index table_name
)
and these will be automatically re-applied when views are updated.
The cascade
option is to refresh materialized views that depend on other
materialized views. For example, say you have materialized view A, which selects
data from materialized view B. To get the most up to date information in view A
you would need to refresh view B first, then right after refresh view A. If you
would like this cascading refresh of materialized views, set cascade: true
when you refresh your materialized view.
Scenic gives you drop_view
too:
def change
drop_view :search_results, revert_to_version: 2
drop_view :materialized_admin_reports, revert_to_version: 3, materialized: true
end
Why do I get an error when querying a view-backed model with find
, last
, or first
?
ActiveRecord's find
method expects to query based on your model's primary key,
but views do not have primary keys. Additionally, the first
and last
methods
will produce queries that attempt to sort based on the primary key.
You can get around these issues by setting the primary key column on your Rails model like so:
class People < ActiveRecord::Base
self.primary_key = :my_unique_identifier_field
end
Why is my view missing columns from the underlying table?
Did you create the view with SELECT [table_name].*
? Most (possibly all)
relational databases freeze the view definition at the time of creation. New
columns will not be available in the view until the definition is updated once
again. This can be accomplished by "updating" the view to its current definition
to bake in the new meaning of *
.
add_column :posts, :title, :string
update_view :posts_with_aggregate_data, version: 2, revert_to_version: 2
When will you support MySQL, SQLite, or other databases?
We have no plans to add first-party adapters for other relational databases at this time because we (the maintainers) do not currently have a use for them. It's our experience that maintaining a library effectively requires regular use of its features. We're not in a good position to support MySQL, SQLite or other database users.
Scenic does support configuring different database adapters and should be extendable with adapter libraries. If you implement such an adapter, we're happy to review and link to it. We're also happy to make changes that would better accommodate adapter gems.
We are aware of the following existing adapter libraries for Scenic which may meet your needs:
Scenic is maintained by Derek Prior and Caleb Thompson, funded by thoughtbot, inc. The names and logos for thoughtbot are trademarks of thoughtbot, inc.
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