Best way to store and organize data in MongoDB - mongodb

Question

I have a users in MongoDB and each user has an interface allowing them to set their current state of hunger being a combination of "hungry", "not hungry", "famished", "starving", or "full"
Each user can enter a multiple options for any period of time. For example, one use case would be "in the morning, record how my hunger is" and the user can put "not hungry" and "full". They can record how their hunger is at any time in the day, and as many times as they want.
Should I store the data as single entries, and then group the data by a date in MongoDB later on when I need to show it in a UI? Or should I store the data as an array of the options the user selected along with a date?

Answer 1

It depends on your future queries, and you may want to do both. Disk space is cheaper than processing, and it's always best to double your disk space than double your queries.
If you're only going to map by date then you'll want to group all users/states by date. If you're only going to map by user then you'll want to group all dates/states by user. If you're going to query by both, you should just make two Collections to minimize processing. Definitely use an array for the hunger state in either case.
Example structure for date grouping:
{ date: '1494288000',
time-of-day: [
{ am: [
{ user: asdfas, hunger-state: [hungry, full] },
{ user: juhags, hunger-state: [full] }
],
pm: [
{ user: asdfas, hunger-state: [hungry, full] },
{ user: juhags, hunger-state: [full] }
]}]}

Answer 2

It depends on how you are going to access it. If you want to report on a user's last known state, then the array might be better:
{
user_id: '5358e4249611f4a65e3068ab',
timestamp: '2017-05-08T17:30:00.000Z',
hunger: ['HUNGRY','FAMISHED'],
}
The timestamps of multiple records might not align perfectly if you are passing in the output from new Date() (note the second record is 99 ms later):
{
user_id: '5358e4249611f4a65e3068ab',
timestamp: '2017-05-08T17:30:00.000Z',
hunger: 'HUNGRY',
}
{
user_id: '5358e4249611f4a65e3068ab',
timestamp: '2017-05-08T17:30:00.099Z',
hunger: ['FAMISHED',
}
You should probably look at your data model though and try to get a more deterministic state model. Maybe:
{
user_id: '5358e4249611f4a65e3068ab',
timestamp: '2017-05-08T17:30:00.000Z',
isHungry: true,
hunger: 'FAMISHED',
}