The following apparently straightforward code caused me quite a bit of grief:

First, the usual 'service' class:

@DataModel
private List<Thing> things;

@DataModelSelection("things")
private Thing selectedThing;

public List<Thing> getThings() {
  ThingRepository repository = new ThingRepository();
  things = repository.getThings();
  return things;
}

public String selectThing() {
  log.info("Selected: " + selectedThing);
  return null;
}

public String selectThing(Thing thing) {
  selectedThing = thing;
  log.info("Selected: " + selectedThing);
}

Followed by an (unsuccessful) attempt to access this by various means within a JSF page.  I tried accessing it via s:link and via  h:commandButton with and without a parameter but only one approach returned 'correct' results.

<h:form>
  <h:dataTable value="#{accountService.things}" var="thing">
    <h:column>
      <f:facet name="header">Name</f:facet>
      <h:outputText id="name" size="10" required="true" value="#{thing.name}"></h:outputText>
    </h:column>
    <h:column>
      <f:facet name="header">Size</f:facet>
      <h:outputText id="size" size="10" required="true" value="#{thing.size}"></h:outputText>
    </h:column>
    <h:column>
      <s:link value="Select" action="#{accountService.selectThing(thing)}" ></s:link>
    </h:column>
    <h:column>
      <h:commandButton value="Select" action="#{accountService.selectThing(thing)}" ></h:commandButton>
    </h:column>
    <h:column>
      <s:link value="Select (dsm)" action="#{accountService.selectThing()}" ></s:link>
    </h:column>
    <h:column>
      <h:commandButton value="Select (dsm)" action="#{accountService.selectThing()}" ></h:commandButton>
    </h:column>
  </h:dataTable>
  <h:commandButton value="Show Choices" action="#{accountService.getThings()}" ></h:commandButton>
</h:form>

The repository spits out the following ordered sequence:

1. One_1

2. Two_22

3. Three_333

4. Four_4444

5. Five_55555

6. Size_666666

Selecting the second, third, fourth and fifth lines (in that order) gave the following surprising (to me at least) results:

• 16:54:43,322 INFO AccountService Selected: null

• 16:54:44,665 INFO AccountService Selected: Three_333 - this is 'correct'

• 16:54:45,931 INFO AccountService Selected: One_1

• 16:54:47,353 INFO AccountService Selected: One_1

Eventually, the errors of my ways was pointed out to me.

Section 27.10.1. of the Seam Reference Guide - Annotations for use with dataTable

states that @DataModel outjects a property of type List, Map, Set or Object{FOOTNOTE DEF  } as a JSF DataModel into the scope of the owning component (or the EVENT scope if the owning component is STATELESS).

Therefore,

@DataModel
private List<Thing> things;

outjects a DataModel context variable called 'things' into the same scope as the bean that it is contained within.

Herein lies the problem.  Attempting to access this data model via:

<h:dataTable value="#{accountService.things}" var="thing">     

refers to a list not to a DataModel.

Changing it to

<h:dataTable value="#{things}" var="thing">

makes it work fine!