Different encoding behavior for PersistDbSpecific with persistent-postgresql and persistent-mysql

[sol]

What is the discrepancy?

persistent-mysql and persistent-postgresql encode PersistDbSpecific differently.

persistent-mysql uses Plain:

persistent/persistent-mysql/Database/Persist/MySQL.hs

Line 239 in 31e25c4

render (P (PersistDbSpecific s)) = MySQL.Plain $ BBS.fromByteString s

However, persistent-postgresql uses Escape (effectively, treating it as a string-like value):

persistent/persistent-postgresql/Database/Persist/Postgresql.hs

Line 554 in 31e25c4

toField (Unknown a) = PGTF.Escape a

When was this introduced?

The behavior for PostgreSQL changed with 9287943.

Why is this a problem?

With the current behavior it is not possible to encode something like point '(2.0,0)' for PostgreSQL.

(I would also be very surprised if the example given in the docs for PersistDbSpecific is still anywhere close working; but I haven't tried that specifically.)

How to address this?

I'm not sure how to best address this. Altering how PersistDbSpecific is encoded would be a breaking change that isn't detected by the type checker. One solution might be to remove PersistDbSpecific and add two new constructors:

• PersistDbSpecificEscape, which uses Escape (current behavior of persistent-postgresql)
• PersistDbSpecificPlain, which uses Plain (current behavior of persistent-mysql)

[parsonsmatt]

#1123 covers this in a PR

In #1122, I outline a proposal for fixing the issue over a few release cycles.

tl;dr: Deprecate PersistDbSpecific in favor of PersistLiteral and PersistLiteralEscaped, basically your suggestion @sol :)