Hi,

You must make a difference between "facts" and other objects (that you can get by "bindings").
"facts" are the objects inserted in the working memory. Only those objects can be matched directly in LHS. Other entities can be accessed by using attributes (of facts) bindings or "from" constructions to get objects from "facts" fields.
And finally, only the "facts" can be modified/inserted/retracted, ie actions that trigger a RETE evaluation and thus new candidate rules to fire.

Here, in your first rule, your LHS is matching facts and use a binding $krankheitsbild for a field of KrankheitsbildVonPatient fact.
And you are modifying the binded attribute, which is not a fact. So it won't trigger RETE evaluation... To me, it should not compile, but apparently it does ....

You can think about 2 corrections :
   1 - make your $krankheitsbild objects real facts, ie insert them in the WM, and don't get them via attribute binding
   2 - do the set on $krankheitsbild, then "update" your containing fact object ( with update($krankheitsbildVonPatient)  )


here are the approximative form of the resulting rules for the 2 approaches :

---------------1-------------------
 I suppose that Krankheitsbild objects are inserted independently, and your model modified like that

declare KrankheitsbildVonPatient
	patient : Patient
end

declare Krankheitsbild
        parent : KrankheitsbildVonPatient
	abdominaleAdipositas : boolean
	adipositasGrad : String
	risikoBegleiterkrankungGrad : int
	risikoMetabolischeKardiovaskulaereKomplikationen : String
	metabolischesSyndrom : boolean
end

so the rule should look like :

rule "...."
        when
                MessdatenVonPatient($patient : patient && (getMessdaten().getBmi() >= 18.5) && (getMessdaten().getBmi() < 25))
                $krankheitsbildVonPatient : KrankheitsbildVonPatient(patient == $patient)
                $krankheitsbild : Krankheitsbild ( parent ==  $krankheitsbildVonPatient )
         then
                modify($krankheitsbild){setAdipositasGrad("Normalgewicht"),
                                                                setRisikoBegleiterkrankungGrad(1)
                                                                }
end



---------------2-------------------
May be simpler, but beware of loops ...

rule "..."
        when
                MessdatenVonPatient($patient : patient && (getMessdaten().getBmi() >= 18.5) && (getMessdaten().getBmi() < 25))
                $krankheitsbildVonPatient : KrankheitsbildVonPatient(patient == $patient && $krankheitsbild : krankheitsbild)
        then
                $krankheitsbild.setAdipositasGrad("Normalgewicht");
                $krankheitsbild.setRisikoBegleiterkrankungGrad(1);
                update ( $krankheitsbildVonPatient  ) ;
end

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