Add support for dynamic GORM valuer registration and retrieval

[iseki0]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Since some bugly database is popular, everyone have to workaround to it.

User Case Description

I have a library https://github.com/iseki0/goda, I don't think it's a better choice to use the GormValuer interface directly, it will make a strong dependency to gorm, and it's unwise since not everyone use gorm. So please give me a chance to register the handler dynamiclly, if someone need, they can register the gorm-enhance-module by the effect import statments, just time time/tzdata.

About the workaround

The MySQL has a very strange behavior:

prepare aa from 'select cast(? as time), ?';
set @a '09:00:00';
execute aa using @a, @a;

+-----------------+----------+
| cast(? as time) | ?        |
+-----------------+----------+
| 00:00:00        | 09:00:00 |
+-----------------+----------+

The problem is on MySQL server, not driver. The Go MySQL driver do paratermized query like the prepare command, I refer to the https://github.com/go-sql-driver/mysql.

The solution is:

prepare aa from 'select cast(cast(? as char) as time)';

Do you have any suggestions? Maybe we should fix it in the MySQL dialect. 毕竟，又不是只有我会倒这个霉，所有传字符串的都会完蛋。

[propel-code-bot]

Dynamic runtime registration of custom GORM valuers

Adds a lightweight mechanism that lets external packages register custom GORM valuers at init time and have them picked up automatically by the core Statement.AddVar() logic. This avoids having third-party libraries depend directly on the Valuer interface while still enabling driver/dialect-specific work-arounds (e.g., MySQL TIME cast issue). The change touches only two files and introduces 23 LoC.

Key Changes

• Created dynamicRegisteredGormValuer map and helper functions RegisterDynamicGormValuerInit() and GetDynamicGormValuer() in valuer.go
• Hooked lookup if valuer := GetDynamicGormValuer(reflect.TypeOf(v)) into statement.go inside AddVar() to translate user values before default handling

Affected Areas

• gorm/valuer.go (new infrastructure)
• gorm/statement.go (value binding path)

This summary was automatically generated by @propel-code-bot

[propel-code-bot]

[CriticalError]

Race condition: The global map dynamicRegisteredGormValuer is written to without synchronization. If multiple packages call RegisterDynamicGormValuerInit concurrently during init() (which can happen when Go initializes packages in parallel), this will cause a data race. While the comment says "shouldn't be called outside the init function", Go doesn't guarantee sequential execution of different packages' init() functions.

Fix: Use sync.Map or protect with sync.RWMutex:

var (
    dynamicRegisteredGormValuer = make(map[reflect.Type]func(context.Context, *DB, any) clause.Expr)
    dynamicValuerMutex sync.RWMutex
)

func RegisterDynamicGormValuerInit(valueType reflect.Type, valuer func(context.Context, *DB, any) clause.Expr) {
    dynamicValuerMutex.Lock()
    defer dynamicValuerMutex.Unlock()
    dynamicRegisteredGormValuer[valueType] = valuer
}

func GetDynamicGormValuer(valueType reflect.Type) func(context.Context, *DB, any) clause.Expr {
    dynamicValuerMutex.RLock()
    defer dynamicValuerMutex.RUnlock()
    return dynamicRegisteredGormValuer[valueType]
}

Context for Agents

**Race condition**: The global map `dynamicRegisteredGormValuer` is written to without synchronization. If multiple packages call `RegisterDynamicGormValuerInit` concurrently during `init()` (which can happen when Go initializes packages in parallel), this will cause a data race. While the comment says "shouldn't be called outside the init function", Go doesn't guarantee sequential execution of different packages' `init()` functions.

**Fix**: Use `sync.Map` or protect with `sync.RWMutex`:

```go
var (
    dynamicRegisteredGormValuer = make(map[reflect.Type]func(context.Context, *DB, any) clause.Expr)
    dynamicValuerMutex sync.RWMutex
)

func RegisterDynamicGormValuerInit(valueType reflect.Type, valuer func(context.Context, *DB, any) clause.Expr) {
    dynamicValuerMutex.Lock()
    defer dynamicValuerMutex.Unlock()
    dynamicRegisteredGormValuer[valueType] = valuer
}

func GetDynamicGormValuer(valueType reflect.Type) func(context.Context, *DB, any) clause.Expr {
    dynamicValuerMutex.RLock()
    defer dynamicValuerMutex.RUnlock()
    return dynamicRegisteredGormValuer[valueType]
}
```

File: valuer.go
Line: 14

[iseki0]

@jinzhu Just a proposal, implementation is a simple demo... Please just ignore the AI reviews😁

[jinzhu]

I don't get what the PR for, maybe you can just use the database/sql's Valuer interface?

[iseki0]

I don't get what the PR for, maybe you can just use the database/sql's Valuer interface?

@jinzhu
The root cause is, on MySQL:

PREPARE aa FROM 'select cast(? as time), ?';
SET @a '09:00:00';
EXECUTE aa USING @a, @a;

The result is:

+-----------------+----------+
| cast(? as time) | ?        |
+-----------------+----------+
| 00:00:00        | 09:00:00 |
+-----------------+----------+

MySQL is bugly, I found a solution:

PREPARE aa FROM 'select cast(cast(? as char) as time)'

I don't know why MySQL is that exactly. Just workaround it,
I consider use your GormValuer interface, but it will make the library hard-dependent to gorm. It's not a good choice.

If the gorm allowing register Valuer dynamically, we can make it like import _ "time/tzdata".