a little closer to understanding Transducers and Transformers...

flatten-with-transducers-test.js

"use strict";

import R from 'ramda';

let userDays = [
  {
    days: {
      day1: {
        billable:   {
          1: 10,
          2: 20,
        },
        unbillable: {
          1: 20,
          3: 30,
        },
      },
      day2: {
        billable:   {
          1: 30,
          4: 40,
        },
        unbillable: {
          1: 40,
          5: 50,
        },
      },
    },
    id:   1
  },
  {
    days: {
      day1: {
        billable:   {
          1: 50,
          6: 60,
        },
        unbillable: {
          1: 60,
          7: 70,
        },
      },
      day2: {
        billable:   {
          1: 70,
          8: 80,
        },
        unbillable: {
          1: 80,
          9: 90,
        },
      },
    },
    id:   2
  },
];

let daysLens = R.lens(R.prop('days'), R.assoc('days'));

/**
 * userInfoObj → dayObject → dayObjectWithUserInfo
 */
let addUserInfoToDay = userInfo => R.map(R.merge(userInfo));

/**
 * [userDays] → [userDays]
 */
let addUserInfoToDays = R.map(userDay => R.over(
  daysLens,
  addUserInfoToDay(R.omit(['days'], userDay)),
  userDay
));

/**
 * a → b
 */
let makeDayKeysUnique = daysObj => R.fromPairs(
  R.map(
    pair => [
      `${ pair[0]}~${ R.prop('id', pair[1]) }`,
      pair[1]
    ],
    R.toPairs(daysObj)
  )
);

/**
 * What's the signature for a transducer?
 */
let transducer = R.compose(
  addUserInfoToDays,
  R.map(R.over(daysLens, makeDayKeysUnique))
);

/**
 * (acc, val) → acc
 *
 * @param acc
 * @param val
 * @returns {Observable|*}
 */
let transformer = (acc, val) => {
  return R.merge(acc, R.prop('days', val));
};

let result = R.transduce(transducer, transformer, {}, userDays);

console.log('result', JSON.stringify(result, null, 2));

/*
result {
  "day1~1": {
    "id": 1,
    "billable": {
      "1": 10,
      "2": 20
    },
    "unbillable": {
      "1": 20,
      "3": 30
    }
  },
  "day2~1": {
    "id": 1,
    "billable": {
      "1": 30,
      "4": 40
    },
    "unbillable": {
      "1": 40,
      "5": 50
    }
  },
  "day1~2": {
    "id": 2,
    "billable": {
      "1": 50,
      "6": 60
    },
    "unbillable": {
      "1": 60,
      "7": 70
    }
  },
  "day2~2": {
    "id": 2,
    "billable": {
      "1": 70,
      "8": 80
    },
    "unbillable": {
      "1": 80,
      "9": 90
    }
  }
}
*/