CODEWITHSUNDEEP
haskell
Vlad the Impala
Vlad the Impala

Reputation: 15872

JSONify a model in Persistent *including* the id

Here's a model I have in Persistent:

Tip
    text Text
    created_at UTCTime
    updated_at UTCTime
    title Text
    category_id CategoryId

And the related ToJSON instance (I couldn't use deriveJSON):

instance ToJSON Tip where
    toJSON (Tip text created_at updated_at title category_id) = object ["text" .= text, "created_at" .= created_at, "updated_at" .= updated_at, "title" .= title, "category_id" .= category_id]

This is almost correct except I want to JSONify the Tip's id in here too...but it's not anywhere on the model! How would I do this? Is there any way to go from an EntityVal to a EntityKey?

Upvotes: 4

Views: 328

Answers (1)

Ben
Ben

Reputation: 71495

An Entity is the pairing of a key and a value. If you just have the value, there's no generic way to know what id it has. If it has any unique constraints you could use that to look up the key.

A given EntityVal could theoretically have been assigned different ids in different backend databases (which might not even all have the same type). Or it could have been manually constructed to be inserted but hasn't actually been inserted yet, so it hasn't been assigned an id. A Tip value doesn't innately have an id; it only maybe has an id in a particular database.

Upvotes: 2

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