schema.buildPolymorphicRelation(relation, field)
	} else if many2many := field.TagSettings["MANY2MANY"]; many2many != "" {
		schema.buildMany2ManyRelation(relation, field, many2many)
	} else if belongsTo := field.TagSettings["BELONGSTO"]; belongsTo != "" {
		schema.guessRelation(relation, field, guessBelongs)
	} else {
		switch field.IndirectFieldType.Kind() {
		case reflect.Struct:
			schema.guessRelation(relation, field, guessGuess)

		reflectValue := association.DB.Statement.ReflectValue
		rel := association.Relationship

		var oldBelongsToExpr clause.Expression
		// we have to record the old BelongsTo value
		if association.Unscope && rel.Type == schema.BelongsTo {
			var foreignFields []*schema.Field
			for _, ref := range rel.References {
				if !ref.OwnPrimaryKey {
					foreignFields = append(foreignFields, ref.ForeignKey)
				}
			}

			return assocDB.Updates(fkNil).Error
		case clause.OpDelete:
			return assocDB.Delete(assocModel).Error
		case clause.OpUpdate:
			return assocDB.Updates(setMap).Error
		}
	case schema.BelongsTo:
		switch op.Type {
		case clause.OpDelete:
			return base.Transaction(func(tx *DB) error {
				assocDB.Statement.ConnPool = tx.Statement.ConnPool
				base.Statement.ConnPool = tx.Statement.ConnPool