I'm not familiar with persistent, but I might have some ideas.

You are using template haskell. So while you are not directly using json, you are using it indirectly, via the code generated by mkPersist [1].
Presumably mkPersist must create an PersistEntity Account instance. Looking at the superclass constraints of PersistEntity [2] we see ToJSON (Key record) and FromJSON (Key record) where the record type variable is you Account type.
You can easily create instances of FromJSON and ToJSON by using Generics [3]. But this only works if you type is an instance of the Generic type class. I think that the code generated by mkPersist depends on the Key Account type having an instance for Generic. Notice that Key is a data family (type level function) and thus Key Account equals some other type.

I can really recommend the -ddump-splices GHC option [4] when debugging code involving template haskell. It is a lot easier to understand a problem when you can just read the generated Haskell code.

1 - https://hackage.haskell.org/package/persistent-template-2.8.0.1/docs/Database-Persist-TH.html#v:mkPersist

2 - https://hackage.haskell.org/package/persistent-2.10.4/docs/Database-Persist-Class.html#t:Persistently

3 - https://hackage.haskell.org/package/aeson-1.4.6.0/docs/Data-Aeson.html#v:parseJSON

4 - https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/debugging.html#ghc-flag--ddump-splices

Op vr 17 jan. 2020 om 18:29 schreef Debasish Ghosh <ghosh.debasish@gmail.com>:

Hi -

I have an entity declaration in persistent as follows:

share [mkPersist sqlSettings { mpsGenerateLenses = True, mpsPrefixFields = False }, mkMigrate "migrateAll"] [persistLowerCase|
Account
accountNo Text
accountType AccountType
accountHolderName Text
accountOpenDate UTCTime default=CURRENT_TIME
accountCloseDate UTCTime Maybe default=NULL
currentBalance MoneyUSD
rateOfInterest Double
Primary accountNo
deriving Show
|]

I am getting this compilation error ..

• No instance for (Generic (Key Account))
arising from the 'deriving' clause of a data type declaration
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
• When deriving the instance for (aeson-1.4.6.0:Data.Aeson.Types.ToJSON.ToJSON
(Key Account))
|
40 | share [mkPersist sqlSettings { mpsGenerateLenses = True, mpsPrefixFields = False }, mkMigrate "migrateAll"] [persistLowerCase|
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

and another exactly similar for FromJSON

What exactly is going wrong here ? I am not using json in the above definition.

regards.
--
Debasish Ghosh