How to handle a structure field modification in templates

Issue

• The user changed a web content structure and moved a nested field to the top level of this structure.
• It produces an error when some articles are rendered.
• Let's provide an example:

• 1. We create an structure with 2 fields:
     • parent
     • child (that is nested in parent)
  2. A new Template is created with this code:

     <p>Parent:${parent.getData()}</p>
     <p>Child:${parent.child.getData()}</p>

  3. A new article (named ARTICLE-1) is created at this point.
  4. The child field is moved to the root of the structure. Now we have 2 sibling fields:
     • parent
     • child
  5. A new article (named ARTICLE-2) is created at this point.
  6. We add 2 web content display portlets in a page, and select both articles (one in each portlet).
  7. Result:
     • ARTICLE-1 is rendered as expected (because the template handles the field names as it needs)
     • ARTICLE-2 produces an error:

       2022-05-10 08:26:37.549 ERROR [http-nio-8080-exec-6][runtime:59] Error executing FreeMarker template
       FreeMarker template error:
       The following has evaluated to null or missing:
       ==> parent.child [in template "20101#20128#35258" at line 10, column 12]
       
       ----
       Tip: It's the step after the last dot that caused this error, not those before it.
       ----
       Tip: If the failing expression is known to legally refer to something that's sometimes null or missing, either specify a default value like myOptionalVar!myDefault, or use <#if myOptionalVar??>when-present<#else>when-missing</#if>. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??
       ----
       
       ----
       FTL stack trace ("~" means nesting-related):
       - Failed at: ${parent.child.getData()} [in template "20101#20128#35258" at line 10, column 10]
       ----
       
       Java stack trace (for programmers):
       ----

Environment

• Liferay DXP 7.2+

Resolution

• The root cause of the error is that changing the structure does not update its articles' content and the field names are different.
• It means that to render it properly we have to handle both cases in the templates:
  • To render the ARTICLE-1 field values we have to use this code in a template:

    <p>Parent:${parent.getData()}</p>
    <p>Child:${parent.child.getData()}</p>

  • To render the ARTICLE-2 field values we have to use this code in a template:

    <p>Parent:${parent.getData()}</p>
    <p>Child:${child.getData()}</p>

• To solve the issue we have 2 possible solutions:
  1. Create 2 templates to handle each case.
  2. Handle this in the same template:
     

     <p>Parent:${parent.getData()}</p>
     
     //original field
     
     <p>
     <#if parent.child?? && parent.child.getData()?? >
     Nested child field value: ${parent.child.getData()}
     <#else>
     parent.child value is null
     </#if>
     </p>
     
     // modifided-moved field
     
     <p>
     <#if child?? && child.getData()?? >
     Modified child filed value: ${child.getData()}
     <#else>
     child value is null
     </#if>
     </p>