How to implement subqueries in cube.js

I'm struggling to see how I would represent the following type of postgres SQL query in a cube.js schema:

SELECT
CASE
WHEN COUNT(tpp.net_total_amount) > 0 THEN
SUM(tpp.net_total_amount) / COUNT(tpp.net_total_amount)
ELSE
NULL
END AS average_spend_per_customer
FROM
(
SELECT
SUM(ts.total_amount) AS net_total_amount
FROM
postgres.transactions AS ts
WHERE
ts.transaction_date >= '2020-11-01' AND
ts.transaction_date < '2020-12-01'
GROUP BY
ts.customer_id,
ts.event_id
) AS tpp
;


I had the feeling that pre-aggregations might be what I'm after, but that doesn't seem to be the case after looking into them. I can get a list of total amount spent per customer per event with the following schema:

cube(`TransactionTotalAmountByCustomerAndEvent`, {
sql: `SELECT * FROM postgres.transactions`,

joins: {

},

measures: {
sum: {
sql: `SUM(total_amount)`,
type: `number`
}
},

dimensions: {
eventId: {
sql: `event_id`,
type: `string`
},

customerId: {
sql: `customer_id`,
type: `string`
},

transactionDate: {
sql: `transaction_date`,
type: `time`
}
},

preAggregations: {
customerAndEvent: {
type: `rollup`,
measureReferences: [sum],
dimensionReferences: [customerId, eventId]
}
}
});


But that is really just giving me the output of the inner SELECT statement grouped by customer and event. How do I query the cube to get the average customer spend per event figure I'm after?

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