JSON instances require an extraneous id field for entities with a Primary

[parsonsmatt]

For a model definition:

share [mkPersist sqlSettings] [persistLowerCase|
JsonEncoding json
    name Text
    age  Int
    Primary name
    deriving Show Eq
|]

This will generate an instance {To,From}JSON (Entity JsonEncoding) that requires an "id" field in the resulting object.

Serializing always puts a blank string.

[parsonsmatt]

This is easy to fix on the ToJSON side of things, but it's tricky on the FromJSON side.

Basically, we're trying to parse:

entityFromJSON :: (PersistEntity e) => Value -> Parser (Entity e)

In the case that entityPrimary (entityDef (Proxy :: Proxy e)) gives us Just fieldDefs, we need to know what fields to parse out. The fieldDefs contain the Haskell name and database names for the primary key fields, but the type is buried in a way that makes it difficult to reify for the parser to know what to pull out. Additionally, it's not necessarily known how to get from the HaskellName to the actual field name - if snake casing or camel casing or whatever was used.

But the real killer here is that the constructor of the Key e is unknown. The only polymorphic key constructor is keyFromValues :: [PersistValue] -> Either Text (Key e), which is insufficient.

For entities with a Primary declaration, we have an implied guarantee that there exists a function keyFromRecord :: PersistEntity rec => rec -> Key rec. When #995 lands, we can guarantee this, but for now, it needs to be a Maybe - keyFromRecordM :: PersistEntity rec => Maybe (rec -> Key rec). This will need to be defined on the type class. Adding a new class member is a breaking change, so it'd (at least) need to be 2.11 before we can meaningfully address this.

Changing a serialization/deserialization format is also a (potentially?) breaking change, so I want to be very careful about changing this in a way that minimizes real world breakage. To figure this out, I need to write some tests that verify how the parser and renderer work.

[parsonsmatt]

Prereq: #1062