code: func SetDefaultPK(fields []*Field) {
  	if HasPK(fields) {
  		return
  	}
  
  	for i, f := range fields {
  		if !f.SQL.IsPrimaryKey && f.SQL.Name == "id" && f.SQL.Type == "int" {
  			fields[i].SQL.IsAutoIncrementing = true
  			fields[i].SQL.AutoIncrement = 1
  			fields[i].SQL.IsRequired = true
  			fields[i].SQL.IsPrimaryKey = true
  			return
  		}
  	}
  }
docs: // If no PK is specified, then set `id` to be PK.