Redux getter if store initial state is empty - facebook

Question

I was watching a talk about Reactjs/Flux during which they showed a code inside Facebook app that is used to preview user profile picture.
Link to the Youtube video.
Snippet:
class ProfilePicture extends React.component{
render(){
let info = usersStore.getUser(this.props.id);
return (<img src={info.uri} />);
}
}
This little snippet had me thinking about how do they implement getUser inside the user store?
I understand that Flux has multiple stores while Flux keeps a single source. Yet such snippet had me thinking that may be.. if I'm fetching a comment for the server the returned value is something like:
/posts
[
{
id:1,
title: 'This is post title',
userId:1,
userName: 'User name',
picture: 'http://user.picture',
}
]
Tet the above ProfilePicture component snippet doesn't read user info from the Post object, it reads it from users Store object, so that made lot sense to me since in my app a user typically have access to <200 other users. So maybe I should stop returning user info with every post. And instead I'd use a user store that store all users info, cache them inlocal storage, retrieve them from server, if needed.
So I'm considering changing my API to respond for posts to:
/post : select id,title from posts;
[
{id:1,title:'Post title'}
]
And have another endpoint that return users info to be loaded lazily by Redux.
So in my Redux I need to change initial state from:
InitialState = {
posts:[] //where all posts and the user who posted this post is stored
}
Into:
InitialState = {
posts:[] //posts and userId only
users:[],//users data <---- here i need a getter function
}
Now one way I can populate the users store is during app initialization. But is this the right timing to do it while app starts up or should I wait until a post component is loaded first, and needs to access data of specific user/s then I'd load the users?
Is it possible for Redux to have getters setters?
class ProfilePicture extends React.component{
render(){
let info = usersStore.getUser(this.props.id);
return (<img src={info.uri} />);
}
}
** Why are they using usersStore.getUser instead of just passing the user by id to the mapStateToProps in the connect function of Redux ?