Hey all, After a long discussion with Aur Saraf, I think we came up with a good approach to join support in Persistent. Let's review the goals: * Allow for non-relational backends, such as Redis (simple key-value stores) * Allow SQL backends to take advantage of the database's JOIN abilities. * Not force SQL backends to use JOIN if they'd rather avoid it. * Keep a simple, straight-forward, type-safe API like we have everywhere else in Persistent. * Cover the most common (say, 95%) of use cases out-of-the-box. So our idea (well, if you don't like it, don't blame Aur...) is to provide a separate module (Database.Persist.Join) which provides special functions for the most common join operations. To start with, I want to handle a two-table one-to-many relationship. For demonstration purposes, let's consider a blog entry application, with entities Author and Entry. Each Entry has precisely one Author, and each Author can have many entries. In Persistent, it looks like: Author name String Asc isPublic Bool Eq Entry author AuthorId Eq title String published UTCTime Desc isPublic Bool Eq In order to get a list of all entries along with their authors, you can use the newly added[1] selectOneMany function: selectOneMany [AuthorIsPublicEq True] [AuthorNameAsc] [EntryIsPublicEqTrue] [EntryPublishedDesc] EntryAuthorEq This will return a value of type: type AuthorPair = (AuthorId, Author) type EntryPair = (EntryId, Entry) [(AuthorPair, [EntryPair])] In addition to Database.Persist.Join, there is also a parallel module named Database.Persist.Join.Sql, which has an alternative version of selectOneMany that is powered by a SQL JOIN. It has almost identical semantics: the only catch comes in when you don't fully specify ordering. But then again, if you don't specify ordering in the first place the order of the results is undefined, so it really *is* identical semantics, just slightly different behavior. Anyway, it's almost 1 in the morning, so I hope I haven't rambled too much. The basic idea is this: Persistent 0.5 will provide a nice, high-level approach to relations. I'll be adding more functions to these modules as necessary, and I'd appreciate input on what people would like to see there. Michael [1] https://github.com/snoyberg/persistent/commit/d2b52a6a7b7a6af6234315492f24f8...