Spring Boot / Thymeleaf - loop within a loop

Question

I have an application with two entities that have a bidirectional one to many relationship. Owners and Bicycles.

So getting the owners via curl will give

[
    {"id":1,
    "userName":"user1",
    "bicycles":
        [
            {
                "id":1,
                "make":"dawes",
                "model":"civic",
                "owner":1
            }
        ]
    },
    {"id":2,
    "userName":"user2",
    "bicycles":
        [
            {
                "id":2,
                "make":"whyte",
                "model":"montpellier",
                "owner":2
            }
            ,{
                "id":4,
                "make":"dahon",
                "model":"tern A7",
                "owner":2
            }
        ]
    } ]

which is fine.

If I create a template which loops in a table,

<table>
    <tr th:each="owner : ${owners}">
      <td th:text="${owner.userName}"></td>
      <td th:text="${owner.bicycles[0].make}"
          th:if="${#lists.size(owner.bicycles)} > 0">"</td>
      <td th:text="${owner.bicycles[0].model}"
          th:if="${#lists.size(owner.bicycles)} > 0"></td> 
    </tr>
  </table>

then I get the expected result in the browser. I realise the above is awful code but I am just interested in getting thymeleaf to work at the moment.

But if I do the following code

<table>
    <tr th:each="owner : ${owners}">
      <td th:text="${owner.userName}"></td>
      <tr th:each="bike : ${owner.bicycles}">
          <td th:text="${bike.make}"></td>
          <td th:text="${bike.model}"></td>  
      </tr>  
    </tr>
  </table>

then I get the following console error

nested exception is org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "owner.bicycles" (template: "nutsthymeleaf" - line 23, col 15)] with root cause

org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'bicycles' cannot be found on null

What I find confusing is that owner.bicycle[index] works. It shows make and model. Yet owner.bicycles appears to be a null field according to the error.

So obviously I am doing something wrong ….

From my point of view also the first solution should give you a Property or field 'bicycles' cannot be found on null exception. Just tried it with an example. Check if bicycles is really not null. — Patrick
– Patrick, Commented Jun 24, 2019 at 13:58
Is that the actual code? The error message is not saying that bicycles doesn't exist, but rather that ${owner} is null, and so it can't get a property of a null object. Are you sure that you didn't accidentally close the loop and ${owner} is out of scope? — Metroids
– Metroids, Commented Jun 24, 2019 at 14:49
@Metroids - I can't see how it goes out of scope. Owner is supposed to be valid to all child tags. Hopefully that includes another each. This has proved valid in other stackoverflow questions. I accept that it is saying owner is null. Is <tr> within a <tr> legal? — John
– John, Commented Jun 24, 2019 at 16:07
Well, typically you'd do that with a <th:block> rather than nested <tr>s, but it should work either way. I'm just saying that the error you are getting can't be explained by the code you have (as far as i can tell). — Metroids
– Metroids, Commented Jun 24, 2019 at 22:22
just before you do return can yo debug to see how the owners attribute that you add on model looks like — want2learn
– want2learn, Commented Jun 25, 2019 at 14:32

John · Accepted Answer · 2019-06-26 09:47:01Z

1

So I found this

Thymeleaf: Getting Property or field cannot be found on null. Iteration of list inside a list

and that has stopped SpelException.

Having played around with it, it does not need 3 levels but just two. The problem arises if a th:each is nested within a th:each. The second th:each has to be in a th:block (or possibly div).

edited Jun 26, 2019 at 9:47

answered Jun 25, 2019 at 7:45

John

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Comments

AmirBll · Accepted Answer · 2019-06-24 18:24:02Z

0

Try if for check bicycles is null or not

<table>
    <tr th:each="owner : ${owners}">
      <td th:text="${owner.userName}"></td>
      <tr th:if={owner.bicycles != null} th:each="bike : ${owner.bicycles}">
          <td th:text="${bike.make}"></td>
          <td th:text="${bike.model}"></td>  
      </tr>  
    </tr>
</table>

answered Jun 24, 2019 at 18:24

AmirBll

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1 Comment

John Over a year ago

Same error but slightly different. Exception evaluating SpringEL expression: "owner.bicycles" (template: "nutsthymeleaf" - line 23, col 49) org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "owner.bicycles" (template: "nutsthymeleaf" - line 23, col 49) And the code is <tr th:if="${owner.bicycles} != null" th:each="bike : ${owner.bicycles}"> So, it seems to accept owner.bicycles in the if but not in the each. And, of course, it accepts owner.bicycles[index].field in earlier code and when finding the length of the list.

maykelange · Accepted Answer · 2023-06-01 11:14:34Z

0

The problem seems to be that you are trying to generate invalid HTML by nesting table row <tr> elements.

To generate the HTML you want, you should use the syntethic th:block tag , which allows you to generate the cells for each bike.

<table>
    <tr th:each="owner : ${owners}">
      <td th:text="${owner.userName}"></td>
      <th:block th:each="bike : ${owner.bicycles}">
          <td th:text="${bike.make}"></td>
          <td th:text="${bike.model}"></td>  
      </th:block>  
    </tr>
  </table>

answered Jun 1, 2023 at 11:14

maykelange

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