简言之，Jenabean至少要求bean作者指明哪个字段保存的值对于该类型的所有实例是惟一的。

To summarize, Jenabean at a minimum requires bean authors to indicate which field holds a value that is unique for all instances of that type.

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因为不可能将目标字段的和作为另一个目标字段的值，所以我们需要求和，并乘以源price字段，作为目标amount字段的值。

Since it is not possible to sum target fields into another target field, we need to sum and multiply the source Price fields into the target Amount field. The XPath expression looks like this.

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如果您要求客户从一系列预先定义的字段值中进行选择，那么选择字段十分有用。

A selection field is useful when you require that a customer select from a series of predefined field values.

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