Hi Michael,

The entity definitions in persistent is very close to the SQL schema, in a 1-to-many relation you must have the foreign key relation defined in the many table.

You should preferably not insert a car before it's owner is inserted, that would give you a null reference. So if possible you should insert people first which will return their id and you can then do the insertion of cars safely. You can also construct keys manually, this is kind of an hack since you may construct invalid IDs.

In a relational schema you can make the name of the person the primary key. There has been some work in adding arbitrarily typed primary keys to persistent, but I'm not sure if it has been released or is on master. Either way, using a person name as a primary key may be a bad idea because of collisions.

Having some mismatch when moving things to relational storage is common. A lot of times I end up creating intermediary types that contain the data in a format that makes it easier to work with. But I don't mind this at all, Haskell makes it very safe to add proxy types and refactor them. You sometimes end up having to do more queries to the DB than seems necessary, but this is only a problem if it turns out to be a bottle neck.

HTH,

Adam

On Thu, Dec 26, 2013 at 12:17 PM, Michael Orlitzky <michael@orlitzky.com> wrote:

I'm loading data from XML into a Haskell data type, and I'd like to use
Persistent to save it to a database. The examples from the Yesod book
have you manually define a FooId field and create the relationships
yourself from the bottom up. For example, "a person has many cars":

blah blah [persistLowerCase|
Person
name String
Car
ownerId PersonId Eq
name String
|]

This works well if you're responsible for creating every person/car
manually. But what if the data are given to you? If I were to parse
people from an XML file, the cars wouldn't have people_ids in them.
Instead I'd get,

blah blah [persistLowerCase|
Person
name String
cars [Car]
Car
name String
|]

As long as the cars list contains another Persistent type, it seems like
I should be able to insert a person and have it insert the cars, with
proper foreign keys, automatically. Doing it manually isn't
straight-forward because I can't add the "ownerId" field to my Car type
and still expect to parse it from the XML (which has no such field).

Any ideas? I'm not married to Persistent yet; I just want to read in
some XML and save it to a database without having to specify the names
and types in three places (preferred place: in Haskell). I don't care
too much about the schema I get as long as it's relational.
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