Add method to check materialized view population

REFRESH MATERIALIZED VIEW CONCURRENTLY doesn't work for a view which is
not populated, even it has unique index. This method is useful to
perform concurrent refresh if it's possible.

Real use case in Rails will be like this:

* Database is loaded from structure.sql to run specs, but the view isn't
  populated yet so concurrent refresh is not possible. We want to
  fallback to normal(non-concurrent) refresh in this case.
* After initial population, views are expected to be refreshed
  concurrently. It's possible because the view is already populated.

Co-authored By: Derek Prior <[email protected]>

lib/generators/scenic/model/templates/model.erb

@@ -1,3 +1,7 @@
   def self.refresh
     Scenic.database.refresh_materialized_view(table_name, concurrently: false, cascade: false)
   end
+
+  def self.populated?
+    Scenic.database.populated?(table_name)
+  end

lib/scenic/adapters/postgres.rb

@@ -222,6 +222,29 @@ def refresh_materialized_view(name, concurrently: false, cascade: false)
         end
       end
 
+      # True if supplied relation name is populated. Useful for checking the
+      # state of materialized views which may error if created `WITH NO DATA`
+      # and used before they are refreshed. True for all other relation types.
+      #
+      # @param name The name of the relation
+      #
+      # @raise [MaterializedViewsNotSupportedError] if the version of Postgres
+      #   in use does not support materialized views.
+      #
+      # @return [boolean]
+      def populated?(name)
+        raise_unless_materialized_views_supported
+
+        sql = "SELECT relispopulated FROM pg_class WHERE relname = '#{name}'"
+        relations = execute(sql)
+
+        if relations.count.positive?
+          relations.first["relispopulated"].in?(["t", true])
+        else
+          false
+        end
+      end
+
       private
 
       attr_reader :connectable

spec/generators/scenic/model/model_generator_spec.rb

@@ -32,5 +32,13 @@ module Scenic::Generators
       expect(model_definition).to contain("self.refresh")
       expect(model_definition).to have_correct_syntax
     end
+
+    it "adds a populated? method to materialized models" do
+      run_generator ["active_user", "--materialized"]
+      model_definition = file("app/models/active_user.rb")
+
+      expect(model_definition).to contain("self.populated?")
+      expect(model_definition).to have_correct_syntax
+    end
   end
 end

spec/scenic/adapters/postgres_spec.rb

@@ -204,6 +204,40 @@ module Adapters
           end
         end
       end
+
+      describe "#populated?" do
+        it "returns false if a materialized view is not populated" do
+          adapter = Postgres.new
+
+          ActiveRecord::Base.connection.execute <<-SQL
+            CREATE MATERIALIZED VIEW greetings AS
+            SELECT text 'hi' AS greeting
+            WITH NO DATA
+          SQL
+
+          expect(adapter.populated?("greetings")).to be false
+        end
+
+        it "returns true if a materialized view is populated" do
+          adapter = Postgres.new
+
+          ActiveRecord::Base.connection.execute <<-SQL
+            CREATE MATERIALIZED VIEW greetings AS
+            SELECT text 'hi' AS greeting
+          SQL
+
+          expect(adapter.populated?("greetings")).to be true
+        end
+
+        it "raises an exception if the version of PostgreSQL is too old" do
+          connection = double("Connection", supports_materialized_views?: false)
+          connectable = double("Connectable", connection: connection)
+          adapter = Postgres.new(connectable)
+          err = Scenic::Adapters::Postgres::MaterializedViewsNotSupportedError
+
+          expect { adapter.populated?("greetings") }.to raise_error err
+        end
+      end
     end
   end
 end