NoSQL不是关系型的

NoSQL==无联接

但我会告诉你一个方法，你可以做你想做的事。

考虑到您的文档没有那个_id。

// Find the User First 

UsersTable.findOne({userID: someuserID}).then((user)=>{
    // You now have the user Object, I hope your userID is unique (it is meant to be)
    //We find all the post by this user
    PostsTable.find({userID: user.userID}).then((posts)=>{
        // Now this posts object contains your array of all the post by the user
        console.log(posts)
    }
}

这就是NoSQL的方式。