arianpotter opened a new issue, #3223:
URL: https://github.com/apache/jena/issues/3223
### Version
5.3.0
### Question
Dear Community Members,
As a part of the project that I am working on now, I have to create dynamic
SPARQL queries. To this end, I am using Jena's ARQ library QueryBuilder.
The issue that I am facing now is that I need to be able to nest
WhereBuilders into each other with FILTER EXISTS and I have a function like
below that adds a WhereBuilder to another as FILTER EXISTS:
` private void addFilterExists(WhereBuilder outerBuilder, WhereBuilder
innerBuilder) {
Element innerElement = innerBuilder.build().getQueryPattern();
ElementGroup group = new ElementGroup();
group.addElement(innerElement);
Expr notExistsExpr = new E_Exists(group);
outerBuilder.addFilter(notExistsExpr);
}`
This works fine if I use it once. But if I have to nest the query again with
this function into another WhereBuilder, and there is a BIND clause somewhere
in the inner query, I get the error "Attempt to assign an expression again".
I can not remove BIND clauses as they are getting added dynamically and are
important. On the other hand, I believe the issue arises from the fact that I
have to build the inner query, and when I put this built query into another
whereclause and then this whereclause gets built as the inner query of another
clause I get this Error while doing: Expr notExistsExpr = new E_Exists(group);
Also, not building the inner query is not an option because trying to nest
these without building(with WhereHandler) leads to a NullPointer exception
Error.
I really appreciate any help or workaround if it is possible, as I am
getting frustrated with this issue and have not found a solution so far.
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