addSponsor: async function (_id) {
    try {
        let val = await companyModel.findById(_id);
        if (val._id == _id) {
            console.log(val); //object is defined!
            this.sponsors.push(val) //object is undefined
        }
    } catch (err) {
        console.log(err)
    }
},

$ node ./mongoose/collections/skaters/models/skaterModel.js
{ _id: 5c489f46d7a825e15d41ae18, name: 'Zero' }
TypeError: Cannot read property 'push' of undefined
    at Object.addSponsor (C:UsersJonasDesktopskaterDBmongoosecollectionsskatersmodelsskaterModel.js:47:31)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:189:7)