Mongodb aggregation:Passing arguments to custom function

Question

Here is my aggregation query

blockColctn.aggregate([{
    "$match": {
        "number": blockNumber
    }

    },
    {
    "$project": {
        "transactions.to": "$transactions.to",
        "transaction": "$transactions",
        "transactions.hash": "$transactions.hash",


    }
    },
    {
    "$lookup": {
        "from": "token_collections",
        "localField": "transactions.to",
        "foreignField": "tokencontractaddress",
        "as": "tokenaccount"
    }
    },
    {
    "$project": {
       "total":getTotal("transactions.hash")
    }
    }])

I am calling getTotal in $project it works fine, however, I would like to pass transactions.hash column value as the function argument. I found a similar issue related to mine Custom functions calculated columns mongodb projection

But it doesn't tell about argument passing

Where are you calling blockColctn.aggregate() from? You can create a composable function that takes in the aggregate and getTotal methods — chridam
– chridam, Commented Jun 25, 2018 at 14:19

chridam · Accepted Answer · 2018-06-26 09:10:16Z

Would start by refactoring the above, first creating a function that takes in the column total value as the function argument and returns the pipeline array:

const getAggregatePipeline = total => ([
    { "$match": {
        "number": blockNumber
    } },
    { "$project": {
        "transactions.to": "$transactions.to",
        "transaction": "$transactions",
        "transactions.hash": "$transactions.hash",
    } },
    { "$lookup": {
        "from": "token_collections",
        "localField": "transactions.to",
        "foreignField": "tokencontractaddress",
        "as": "tokenaccount"
    } },
    { "$project": { total } }
])

Now you can pass this on to the aggregate method as

blockColctn.aggregate(getAggregatePipeline(getTotal("transactions.hash")))

As you can see we're just nesting functions inside each other so that when they're called with a final value, the result explodes outwards through each layer. This can be further refactored to create a composition of all those functions (i.e. that returns a new function) with the following blueprint

const compose = (f1, f2, f3) => value => f1( f2( f3(value) ) )

You can then call this as

const agg = compose(blockColctn.aggregate, getAggregatePipeline, getTotal)("transactions.hash")

For a compose function that’s fully variadic, i.e. one that can take an arbitrarily long list of functions and then work right to left (inner to outer):

const compose = (...fns) => fns.reduce((f, g) => (...args) => f(g(...args)))

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