Simplify setting tsvector values

We make the code easier to read and at the same time we remove a SQL
injection false positive regarding the use of `WHERE id = #{id}`.

We still get a warning about SQL injection regarding the `tsv =` part.
It's a false positive, since the value of that parameter does not
depend on user input.

app/models/concerns/search_cache.rb

@@ -6,8 +6,7 @@ module SearchCache
   end
 
   def calculate_tsvector
-    ActiveRecord::Base.connection.execute("
+    self.class.where(id: id).update_all("tsv = (#{searchable_values_sql})")
-      UPDATE #{self.class.table_name} SET tsv = (#{searchable_values_sql}) WHERE id = #{id}")
   end
 
   private