Spring MVC - Drop Down Object Selection - No primary identifier

Question

A fairly common use case occurs where there is a list of Java objects, from which selections can be made on a web form - usually you'd use the primary key of the object as the value so that the controller could either do a lookup, or just bind the key to whichever object is created/updated.

My problem is that the list to choose from are not persistent, keyed objects, they are business models from a service which have no reasonable way to retrieve them based on the data contained. Below is some psuedo code where a list of Foo's are given to the page, and we can easily communicate to the controller onSubmit the name of Foo, but what if there are other fields of Foo that need to be submitted?

controller:

referenceData() {
    ...
    List foos = fooService.getFoosForBar( bar )
    return { 'foos', foos }
}

jsp:

<form>
   ...
<spring:bind path="formData.foo">
    <select name="<c:out value="${status.expression}" />">
        <c:forEach items="${foos}" var="foo">
            <option value="<c:out value="${foo.name}"/>">
                <c:out value="${foo.name}"/>
            </option>
        </c:forEach>
    </select>
</spring:bind>
   ...
</form>

Some example solutions would be to use hidden fields to submit Foo's other properties and keep them in sync as the selection is changed, but I prefer not to use JavaScript in a situation like this where it will likely add confusion. There are certainly other ways to accomplish this too.

My question is does there exist any standard practice for accomplishing this? Or should I just come up with my own way of doing so? I'd rather not re-invent wheels if possible, and this is so seemingly common that just winging it may not be the best approach.

DwB · Accepted Answer · 2017-09-22T14:10:16.923

Based on your limitations, you must encode the other data memebers of foos as the value of the option.
<option label="${foo.name}" value="${foo.encodedValues}"/>
The encodedValues method might look something like this:


    private String SEPARATOR = ",";

    public String getEncodedValues()
    {
        final StringBuffer returnValue = new StringBuffer();

        returnValue.append(field1);
        returnValue.append(SEPARATOR);
        returnValue.append(field2);
        returnValue.append(SEPARATOR);
        returnValue.append(field3);

        return returnValue.toString();
    }

If you have a number of selects that need to have encoded values, you may want to create a class that does the encoding and decoding of these values to centralize the code.

I have used this approach by registering a custom binder in the controller to go from the string to the representative object (that's the centralized code that dwb suggests). — Nate, Nov 11 '10 at 21:19

score 3 · Answer 2 · answered Nov 11 '10 at 14:53

3

You can use the index of the element in the list to get it back in the POST request.

<spring:bind path="formData.fooIndex">
  <select name="<c:out value="${status.expression}" />">
    <c:forEach items="${foos}" var="foo" varStatus="i">
        <option value="<c:out value="${i.index}"/>">
            <c:out value="${foo.name}"/>
        </option>
    </c:forEach>
  </select>
</spring:bind>

In your POST handler, use foos.get(formData.getFooIndex()) If foos can change between the GET and POST requests, you need to put foos in session so that you definitely reference the same object in your POST handler as you did in the GET handler.

answered Nov 11 '10 at 14:53

pjesi

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I can't get the list again and expect it to be the same, putting it in the session is not ideal - if the form isn't posted, and even if it were, it's possible the list would just stay in the session until the session ended. What else could I do to allow the post handler to have the list? Request is too short term, but the session is too long term – walnutmon Nov 11 '10 at 17:48
Then you can marshal your object in the value. In your POST handler, you unmarshal it back to the original value from the service. "> – pjesi Nov 12 '10 at 10:20

Spring MVC - Drop Down Object Selection - No primary identifier

2 Answers2