Is it possible to join one field of an array unwind onto the unwound array? - mongodb

Question

Pretty new to mongo and haven't been able to figure out how to perform a query.
I have an accounts collection that looks like this:
{
"_id" : ObjectId("1"),
"time" : ISODate("2018-10-20T05:57:15.372Z"),
"profileId" : "1",
"totalUSD" : "1015.5513030613",
"accounts" : [
{
"_id" : ObjectId("2"),
"accountId" : "1",
"currency" : "USD",
"balance" : "530.7934159683763000",
"available" : "530.7934159683763",
"hold" : "0.0000000000000000",
"exchangeRateUSD" : "1"
},
{
"_id" : ObjectId("5"),
"accountId" : "4",
"currency" : "BTC",
"balance" : "0.0759214200000000",
"available" : "0.07592142",
"hold" : "0.0000000000000000",
"exchangeRateUSD" : "6384.995"
},
],
}
I store only exchangeRateUSD for each currency, and not exchangeRateXXX where XXX is currency name, because there can be an arbitrary number of currencies and currency pairs. But when I query the accounts collection it will always be queried by a currency pair, eg: BTC-USD. Keeping it simple for now, I can assume the currency pair will always be XXX-USD.
When I query the accounts collection I'd like to add a 'virtual' field to each account object: exchangeRateCrypto and then on the top-level accounts document I'd like to add totalCrypto which would just be the total account value in the given crypto. Eg: USD account balance * exchangeRateCrypto + crypto account balance * exchangeRateCrypto (which would equal 1).
My current query without the exchangeRateCrypto and totalCrypto looks like:
db.accounts.aggregate([
{ $unwind: '$accounts' },
{ $match: { 'accounts.currency': { $in: [ 'USD', 'BTC' ] }}},
{
$group: {
_id: '$_id',
time: { $first: '$time' },
profileId: { $first: '$profileId' },
accounts: { $push: '$accounts' },
totalUSD: { $sum: { $multiply: [ { $toDouble: '$accounts.balance' }, { $toDouble: '$accounts.exchangeRateUSD' } ] } }
}
}
]);
I'm trying to figure out how to 'reach' into the BTC row and calculate the exchangeRateCrypto by simply doing 1 / exchangeRateUSD and then projecting/returning the accounts document and subdocument as:
{
"_id" : ObjectId("1"),
"time" : ISODate("2018-10-20T05:57:15.372Z"),
"profileId" : "1",
"totalUSD" : "1015.5513030613",
"totalCrypto" : "0.1590527953", // 530.7934159683763 * 0.0001566171939 + 0.07592142 * 1
"accounts" : [
{
"_id" : ObjectId("2"),
"accountId" : "1",
"currency" : "USD",
"balance" : "530.7934159683763000",
"available" : "530.7934159683763",
"hold" : "0.0000000000000000",
"exchangeRateUSD" : "1",
"exchangeRateCrypto" : "0.0001566171939", // 1 / 6384.995
},
{
"_id" : ObjectId("5"),
"accountId" : "4",
"currency" : "BTC",
"balance" : "0.0759214200000000",
"available" : "0.07592142",
"hold" : "0.0000000000000000",
"exchangeRateUSD" : "6384.995",
"exchangeRateCrypto" : "1"
},
],
}
but haven't been able to figure out a good way of doing this.
It seems it should be pretty straightforward, but still learning Mongo.
Any tips?
Thanks!

Answer 1

The solution might be a bit long and probably it can be shortened however I want you to understand proposed way of thinking step by step.
var secondCurrency = "BTC";
var secondCurrencyFieldName = "exchangeRate" + secondCurrency;
var secondCurrencyFieldNameRef = "$" + secondCurrencyFieldName;
var totalFieldName = "total" + secondCurrency;
db.accounts.aggregate([
{ $unwind: "$accounts" },
{ $match: { "accounts.currency": { $in: [ "USD", secondCurrency ] }}},
{
$group: {
_id: "$_id",
time: { $first: "$time" },
profileId: { $first: "$profileId" },
accounts: { $push: "$accounts" },
totalUSD: { $sum: { $multiply: [ { $toDouble: "$accounts.balance" }, { $toDouble: "$accounts.exchangeRateUSD" } ] } }
}
},
{
$addFields: {
[secondCurrencyFieldName]: {
$filter: {
input: "$accounts",
as: "account",
cond: { $eq: [ "$$account.currency", secondCurrency ] }
}
}
}
},
{
$addFields: {
[secondCurrencyFieldName]: {
$let: {
vars: { first: { $arrayElemAt: [ secondCurrencyFieldNameRef, 0 ] } },
in: { $toDouble: "$$first.exchangeRateUSD" }
}
}
}
},
{
$addFields: {
accounts: {
$map: {
input: "$accounts",
as: "account",
in: {
$mergeObjects: [
"$$account",
{
[secondCurrencyFieldName]: {
$cond: [ { $eq: [ "$$account.currency", secondCurrency ] }, 1, { $divide: [ 1, secondCurrencyFieldNameRef ] } ]
}
}
]
}
}
}
}
},
{
$addFields: {
[totalFieldName]: {
$reduce: {
input: "$accounts",
initialValue: 0,
in: {
$add: [
"$$value",
{ $multiply: [ { $toDouble: "$$this.balance" }, "$$this." + secondCurrencyFieldName ] }
]
}
}
}
}
}
]).pretty()
So we can start with $addFields which can either add new field to existing document or repace existing field. After the $group stage you have to find the USD-XXX exchange rate (using $filter and $let + $arrayElemAt in the next pipeline stage). Having this value you can use $addFields again combined with $map and $mergeObjects to add new field to nested array and that field will represent the ratio between USD and XXX currency. Then you can use $addFields again with $reduce to get the total of all accounts for XXX currency.
Output:
{
"_id" : ObjectId("5beeec9fef99bb86541abf7f"),
"time" : ISODate("2018-10-20T05:57:15.372Z"),
"profileId" : "1",
"accounts" : [
{
"_id" : ObjectId("5beeec9fef99bb86541abf7d"),
"accountId" : "1",
"currency" : "USD",
"balance" : "530.7934159683763000",
"available" : "530.7934159683763",
"hold" : "0.0000000000000000",
"exchangeRateUSD" : "1",
"exchangeRateBTC" : 0.00015661719390539853
},
{
"_id" : ObjectId("5beeec9fef99bb86541abf7e"),
"accountId" : "4",
"currency" : "BTC",
"balance" : "0.0759214200000000",
"available" : "0.07592142",
"hold" : "0.0000000000000000",
"exchangeRateUSD" : "6384.995",
"exchangeRateBTC" : 1
}
],
"totalUSD" : 1015.5513030612763,
"exchangeRateBTC" : 6384.995,
"totalexchangeRateBTC" : 0.15905279535242806
}