Moveit: generating a motion plan fixing the state of a joint

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I would need to keep one joint fixed in a moveit arm motion plan. I know that is possible creating different moveGroups that excludes some joints but the idea in this case is using the actiive movegroup that also contains the target fixed joint.

I tried to do that using JointContraints but it did not work, something like this:

    jc.joint_name = "torso_lift_joint";  
    jc.position = getCurrentTorsoValue;
    jc.tolerance_above = 0.05;
    jc.tolerance_below = 0.05;
    jc.weight = 1.0; 

    path_constraints.joint_constraints.push_back(jc);

    moveGroupSmaccClient_->moveGroupClientInterface.setPathConstraints(path_constraints);

How could I generate a moveit arm motion plan with some joints keeping a fixed value?