mongodb won't find my schema method in nested container

I am trying to access the method of a schema which is stored inside a mixed container. Here is the situation : I have some cases model which can be many different things, so I have a schema for each of these things which are stored in the "caseContent" mixed property.

var CaseSchema = mongoose.Schema({
    caseContent : {},
    object : {type:String, default : "null"},
    collision : {type : Boolean, default : false}
});

The caseContent property is then filled with the model of one of my schemas, like this one for exemple :

var TreeSchema = new mongoose.Schema({
    appleCount : {type : Number, default : 3}
});
TreeSchema.methods.doStuff = function (data) {
    console.log('Hey, listen');
    return true;
};

Then, I want to use the method of my schema from the original container :

CaseSchema.methods.doStuff = function (data) {
    if (this.caseContent.doStuff !== undefined) {
        this.caseContent.doStuff();
                    console.log('it worked');
    } else {
        console.log('doStuff is undefined');
        console.log(this.caseContent.doStuff);
    }
};

On the first time (when everything is added on the database) it works. Then, the caseContent.doStuff seems to be always undefined (the console.log('doStuff is undefined'); appears each time).

So I think there is something that keeps me from calling that method probably because of the mixed type of the container... Is there any workarround for that ?