Versioned database views for Rails
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Latest commit 13709f4 Dec 21, 2016 @devonestes devonestes committed with derekprior Address further comments in cascading materialized view refresh (#180)
Derek had a few other comments about this feature that didn't get
addressed before it was merged, so this PR is to address those issues. It aims
to make the process of parsing the dependency information from Postgres a
little easier to understand, addresses some style issues, and also adds
documentation.

README.md

Scenic

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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.

Great, how do I create a view?

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

Cool, but what if I need to change that view?

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.

What if I want to change a view without dropping it?

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.

Can I use this view to back a model?

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

What about materialized views?

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.

I don't need this view anymore. Make it go away.

Scenic gives you drop_view too:

def change
  drop_view :search_results, revert_to_version: 2
end

FAQs

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?

We have no plans to add first-party support for MySQL at this time because we (the maintainers) do not currently have a use for it. 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 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.

About

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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