yesodweb/persistent

newtype interferes with unique constraints in MySQL

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#100 opened on Oct 29, 2012

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 (11 comments) (0 reactions) (0 assignees)Haskell (302 forks)auto 404
Feature RequestMySQLgood first issue

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Description

Text fields can have unique constraints over them.

However, when a Text field is newtype'd, for extra type safety, this facility appears to break:

In Model.hs with GeneralizedNewtypeDeriving:

newtype Hash = Hash { unHash :: Text }
    deriving (Show, Read, Eq, PathPiece, PersistField)

In config/models:

Profile
  hash Hash
  UniqueHash hash

On initial database migration with mysql, we see:

Migrating: ALTER TABLE `profile` ADD CONSTRAINT `unique_hash` UNIQUE(`hash`)
hpd3js: ConnectionError {errFunction = "query", errNumber = 1170, errMessage = "BLOB/TEXT column 'hash' used in key specification without a key length"}

Uncaught instance of overlapping instances floating around, perhaps? (Am I missing a deriving statement?)

Contributor guide

Research direction
Examine the Template Haskell code that generates unique constraints for newtype fields, focusing on column type detection and key length handling for MySQL.
Tech stack
haskell
Domain
backenddatabase
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
HaskellMySQLPersistent
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40