Why does Mongoose want to cast my string to ObjectId?

I have this shema:

S_logs =  new mongoose.Schema({
    user_ip : {type: mongoose.Schema.Types.ObjectId},
    user_id : String,
    user_agent : String,
    canal_id :  String,
    theme_id :  String,
    video_id :  String,
    osef : String,
    time : Number,
    action: String,
    is_newuser : String,
    operator : String,
    template : String,
    catalogue : String,
    referer : String,
    from : String,
    osef1 : String
});

That I fill like this:

while (i < logs.length) {
    DB_logs[i] = new Model({

        user_ip : logs[i].user_ip,
        user_id : logs[i].user_id,
        user_agent : logs[i].user_agent,
        canal_id :  logs[i].canal_id,
        theme_id :  logs[i].theme_id,
        video_id :  logs[i].video_id,
        osef : logs[i].osef,
        time : logs[i].time,
        action: logs[i].action,
        is_newuser : logs[i].is_newuser,
        operator : logs[i].operator,
        template : logs[i].template,
        catalogue : logs[i].catalogue,
        referer : logs[i].referer,
        from : logs[i].from,
        osef1 : logs[i].osef1
    });
    i = i + 1;
}

There is no _id field, and I don t understand what use it have, but mongoose seems to want user_ip to be casted to ObjectId, that I do not want, user_ip is a data field, not a index!

How can I either:

-Tell mongoose that I do not want a _id
-Auto generate a id for each object?

And what did I do wrong so that mongoose took a data field for a id?

{ message: 'Cast to ObjectId failed for value "157.55.39.208" at path "user_ip"',
  name: 'CastError',
  type: 'ObjectId',
  value: '157.55.39.208',
  path: 'user_ip' }