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  • Using Select With a List

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hiddentrue

Using SelectModel, SelectModelFactory and ValueEncoder for Select menus populated from a database

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The documentation for the Select Component and the Tapestry Tutorial provide simplistic examples of populating a drop-down menu (as the (X)HTML Select element) using comma-delimited strings and enums. However, most real-world Tapestry applications need to populate such menus using values from a database, commonly in the form of java.util.List objects. Doing so generally requires a SelectModel and a ValueEncoder bound to the Select component with its "model" and "encoder" parameters:

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In the above example, ColorSelectModel must be of type SelectModel, or anything that Tapestry knows how to coerce into a SelectModel, such as a List or a Map or a "value=label,value=label,..." delimited string, or anything Tapestry knows how to coerce into a List or Map, such as an Array or a comma-delimited String.

SelectModel

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    *JumpStart Demos:*
    [Total Control Object Select|http://jumpstart.doublenegative.com.au/jumpstart/examples/select/totalcontrolobject]
    [ID Select|http://jumpstart.doublenegative.com.au/jumpstart/examples/select/id]
    [Easy ID Select|http://jumpstart.doublenegative.com.au/jumpstart/examples/select/easyid]
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A SelectModel is a collection of options (specifically OptionModel objects) for a drop-down menu. Basically, each option is a value (an object) and a label (presented to the user).

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In addition to a SelectModel, your Select menu is likely to need a ValueEncoder. While a SelectModel is concerned only with how to construct a Select menu, a ValueEncoder is used when constructing the Select menu and when interpreting the encoded value that is submitted back to the server. A ValueEncoder is a converter between the type of objects you want to represent as options in the menu and the client-side encoded values that uniquely identify them, and vice-versa.

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{float:right|background=#eee|padding=0 1em}
    *JumpStart Demo:*
    [Easy Object Select|http://jumpstart.doublenegative.com.au/jumpstart/examples/select/easyobject]
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Most commonly, your ValueEncoder's toClient() method will return a unique ID (e.g. a database primary key, or perhaps a UUID) of the given object, and its toValue() method will return the object matching the given ID by doing a database lookup (ideally using a service or DAO method).

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