在Groundhog Haskell中通过投影键选择行的理想方法是什么

Question

I'm trying to create a many to many table using Groundhog in Haskell, which basically looks like this if I cut away all the other logic I have:

data FooRow = FooRow {
  fooRowUUID :: UUID
}
deriving instance Show FooRow

data BarRow = BarRow {
  barRowUUID :: UUID
}
deriving instance Show BarRow

data FooToBarRow = FooToBarRow {
  fooToBarRowUUID :: UUID,
  fooToBarRowFoo :: DefaultKey FooRow,
  fooToBarRowBar :: DefaultKey BarRow
}
deriving instance Show FooToBarRow

Now, trying to define operations, I can get and insert all of these records just fine, however I'm not sure how to go from having a FooRow, with it's ID, and then get all the related BarRows by way of the many to many table. Right now I've played with something like this:

getBarsForFoo fooID = do
  barKeys <- project 
    (FooToBarRowBarField)
    (FooToBarRowFooField ==. (Foo_FooKey fooID))
  select $ (BarRowUUIDField `in_` barKeys)

However this doesn't typecheck, with the error:

Couldn't match type 'UUID' with 'BarRow'

Inspecting just the results of the project with putStrLn, I can see that the type of barKeys is:

[Bar_BarKey UUID]

but I don't quite understand how to make use of that within my query. I don't see any examples like this in the Groundhog documentation, so I'm hoping someone will be able to put me on the right path here.

Answer 1

I'm quite certain there are more efficient ways to go about this (there's going to be a bunch of underlying queries with this approach), but this does at at least get the job done for now.

getBarsForFoo fooID = do
    barKeys <- project 
        (FooToBarRowBarField)
        (FooToBarRowFooField ==. (Foo_FooKey fooID))
    q <- mapM (getBy) barKeys
    return (catMaybes q :: [BarRow])