skip updating created_at column while using gorm Update with associations - go-gorm

Question

Background
We are using gorm association as we have nested hierarchy. We are using Create() for creation of records across multiple tables and Updates opeartion for updating the records.
While doing the update, we first fetch complete hierarchy, update it and then invoke update().
Issue
During the update we do not want to change the created_at timestamp.
For top level struct/ record, this is working fine and created_at column is not updated. However, for nested records created_at timestamp is changed always on performing update operation. On update, we just want to update the updated_at column and keep created_at intact.
Structs:
struct A {
ID string `gorm:"column:id;type:char(36); not null;primary_key"`
B []*B `gorm:"foreignKey:AID;references:ID"`
CreatedAt time.Time `gorm:"column:created_at;type:timestamp; not null"`
UpdatedAt time.Time `gorm:"column:updated_at;type:timestamp; not null"`
}
struct B {
ID string `gorm:"column:id;type:char(36); not null;primary_key"`
AID string `gorm:"column:aid;type:char(36); not null"`
C []*C `gorm:"foreignKey:BID;references:ID"`
CreatedAt time.Time `gorm:"column:created_at;type:timestamp; not null"`
UpdatedAt time.Time `gorm:"column:updated_at;type:timestamp; not null"`
}
struct C {
ID string `gorm:"column:id;type:char(36); not null;primary_key"`
BID string `gorm:"column:bid;type:char(36); not null"`
CreatedAt time.Time `gorm:"column:created_at;type:timestamp; not null"`
UpdatedAt time.Time `gorm:"column:updated_at;type:timestamp; not null"`
}
Below is the snippet of code how we are updating the records:
txn.Session(&gorm.Session{Context: ctx, FullSaveAssociations: true}).
Updates(record)
Query Executed:
UPDATE A SET updated_at=? WHERE id = ?
INSERT INTO B (id, aid, created_at, updated_at) ON DUPLICATE KEY UPDATE id=VALUES(id), aid=(aid),created_at=VALUES(),updated_at=VALUES(updated_at)
INSERT INTO C (id, bid, created_at, updated_at) ON DUPLICATE KEY UPDATE id=VALUES(id), bid=(aid),created_at=VALUES(),updated_at=VALUES(updated_at)
Things tried:
Omit: we have tried using db.omit("B.CreatedAt, B.C.CreatedAt"). However, this is not working.
<-:create: This leads to failure in Unit tests with reason: there is a remaining expectation which was not matched. Essentially, nested records are not persisted.