Add ability to override multisearch rebuild SQL

Now you can implement your own rebuild method to handle models that use
dynamic method calls to populate their content. This is the only way for
PgSearch::Multisearch.rebuild to work with a model that uses non-column
methods in its multisearchable :against option.

README.rdoc

@@ -129,6 +129,39 @@ Currently this is only supported for :against methods that directly map to Activ
  PgSearch::Document.delete_all(:searchable_type => "Ingredient")
  Ingredient.find_each { |record| record.update_pg_search_document }
 
+
+You can also provide a custom implementation for rebuilding the documents by adding a class method called `rebuild_pg_search_documents` to your model.
+
+ class Movie < ActiveRecord::Base
+   belongs_to :director
+
+   def director_name
+     director.name
+   end
+
+   multisearchable against: [:name, :director_name]
+
+   # Naive approach
+   def self.rebuild_pg_search_documents
+     find_each { |record| record.update_pg_search_document }
+   end
+
+   # More sophisticated approach
+   def self.rebuild_pg_search_documents
+     connection.execute <<-SQL
+      INSERT INTO pg_search_documents (searchable_type, searchable_id, content, created_at, updated_at)
+        SELECT 'Movie' AS searchable_type,
+               movies.id AS searchable_id,
+               (movies.name || ' ' || directors.name) AS content,
+               now() AS created_at,
+               now() AS updated_at
+        FROM movies
+        LEFT JOIN directors
+          ON directors.id = movies.director_id
+     SQL
+   end
+ end
+
 ==== Disabling multi-search indexing temporarily
 
 If you have a large bulk operation to perform, such as importing a lot of records from an external source, you might want to speed things up by turning off indexing temporarily. You could then use one of the techniques above to rebuild the search documents off-line.

lib/pg_search/multisearch.rb

@@ -16,7 +16,11 @@ class << self
       def rebuild(model, clean_up=true)
         model.transaction do
           PgSearch::Document.where(:searchable_type => model.name).delete_all if clean_up
-          model.connection.execute(rebuild_sql(model))
+          if model.respond_to?(:rebuild_pg_search_documents)
+            model.rebuild_pg_search_documents
+          else
+            model.connection.execute(rebuild_sql(model))
+          end
         end
       end
 

spec/pg_search/multisearch_spec.rb

@@ -24,7 +24,7 @@
     end
 
     it "should operate inside a transaction" do
-      model.should_receive(:transaction).once()
+      model.should_receive(:transaction).once
 
       PgSearch::Multisearch.rebuild(model)
     end
@@ -70,6 +70,27 @@
           PgSearch::Document.count.should == 2
         end
       end
+
+      context "when the model implements .rebuild_pg_search_documents" do
+        before do
+          def model.rebuild_pg_search_documents
+            connection.execute <<-SQL
+              INSERT INTO pg_search_documents
+                (searchable_type, searchable_id, content, created_at, updated_at)
+                VALUES
+                ('Baz', 789, 'baz', now(), now());
+            SQL
+          end
+        end
+
+        it "should call .rebuild_pg_search_documents and skip the default behavior" do
+          PgSearch::Multisearch.should_not_receive(:rebuild_sql)
+          PgSearch::Multisearch.rebuild(model)
+
+          record = PgSearch::Document.find_by_searchable_type_and_searchable_id("Baz", 789)
+          record.content.should == "baz"
+        end
+      end
     end
 
     describe "inserting the new documents" do