createPost(parent,args,ctx,info){
        const userExists = ctx.users.some((user)=> user.id === args.author)

        if(!userExists){
            throw new Error('User does not exist!')
        }

        const post = {
            id: uuidv4(),
            ...args
        }

         ctx.posts.push(post)
        if(args.published === true){
            pubsub.publish('post', {
                post:{
                    mutation: 'CREATED',
                    data: post
                }
            })
        }
        return post
    }