/**
 *
 */
const checkReference = ( document, path ) => document.get( path ).constructor.name === "DocumentReference";

/**
 *
 */
const getReferences = document => Object.keys( document.exists ? document.data() : {} ).filter( path => checkReference( document, path ) );

/**
 *
 */
const populate = document => {

    const references = getReferences( document )

    if ( references == 0 ) return document

    return Promise
        .all( references.map( reference => document.get( reference ).get().then( snapshot => document[ reference ] = snapshot ) ) )
        .then( () => references.forEach( reference => populate( document[ reference ] ) ) )
}