fix: preload shouldn't overwrite the value of join

[black-06]

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

What did this pull request do?

Fix the issue #6715, preload shouldn't overwrite the value of join.

See #6715 (comment)

User Case Description

type (
	Preload struct {
		ID       uint
		Value    string
		NestedID uint
	}
	Join struct {
		ID       uint
		Value    string
		NestedID uint
	}
	Nested struct {
		ID       uint
		Preloads []*Preload
		Join     Join
		ValueID  uint
	}
	Value struct {
		ID     uint
		Name   string
		Nested Nested
	}
)

DB.Joins("Nested").Joins("Nested.Join").Preload("Nested.Preloads").First(&find)

[alexisvisco]

Hello, just to be sure, we are waiting a merge from @jinzhu ?

[black-06]

Hello, just to be sure, we are waiting a merge from @jinzhu ?

I think so 😴