Fix/preload composite foreign keys

[kankankanp]

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

What did this pull request do?

Fix LookUpField to prevent returning wrong embedded fields when using namer-based column lookup.

When searching for a field like "UserID" in a schema with embedded structs, the namer-based lookup (added in #7619) would incorrectly return embedded fields with matching DBNames. For example, searching for "UserID" would return an embedded "ID" field with DBName "user_id" instead of returning nil.

This fix adds a check to ensure the namer-based lookup only returns a field if its Name exactly matches the search name.

Changes:

• schema/schema.go: Add name matching check in LookUpField
• schema/schema_test.go: Add regression test TestLookUpFieldWithEmbeddedStruct

User Case Description

Using composite foreign keys with embedded structs causes Preload to fail in v1.31.1.

type SlingUser struct {
    ID    int64      `gorm:"primaryKey"`
    SrcID string     `gorm:"primaryKey"`
    User  EmbeddedUser `gorm:"embedded;embeddedPrefix:user_"`
}

type SlingShift struct {
    ID     string `gorm:"primaryKey"`
    SrcID  string `gorm:"primaryKey"`
    UserID int64
    User   *SlingUser `gorm:"foreignKey:SrcID,UserID;references:SrcID,ID"`
}

// This fails in v1.31.1, works in v1.31.0
db.Preload("User").First(&shift)
// ERROR: User is nil - Preload failed!

Fixes #7686

[kankankanp]

@jinzhu
I've submitted a PR. Could you please take a look when you have a moment?

[arraytad]

I can confirm that building against this PR, in addition to fixing the testcase from #7686 also fixes my issue in the production codebase that I extracted the testcase from. Thank you for such a fast review + fix! 💚