Pass attributes and other mappings to queries

[jgaskins]

If we pass attributes defined in the mapper to the queries, this will allow us to do several different things:

• Convert from given attribute on the object to how it is stored in the database a la Allow mapping object attributes to different DB fields #15.
  • This is an important one for Data Mappers to implement. It's one of the primary reasons to use a Data Mapper over an Active Record.
  • This also makes database migrations a lot less of a necessity since if you need to rename something on your model, there isn't a corresponding database change.
• Determine how to serialize different types of queries
  • Given the following query on a Postgres database: mapper.select { |article| article.author.name == 'Jamie' }
  • If the author attribute is referenced, we could do a JOIN and not only get that article, but side-load the author, as well.
  • If the author is embedded, we would know that it is serialized as JSON and use the appropriate operators to dig into it and retrieve its name attribute.
  • This could also extend to doing things like this to find articles with specific tags: mapper.select { |article| article.tags.any? { |tag| tag == 'perpetuity' } }
• Passing the class->table mapping will become important once we support mapping to a different table than the class name, as well, so when we do JOIN queries we're able to join the right table.