Aggregate inside expression (e.g. `coalesce(count(...), 0)`) throws `QueryCompilationError: Unknown expression type: agg`

[evanheckert]

Summary

Wrapping an aggregate in another expression (e.g., coalesce(count(item.property), 0)) throws during compilation. Using the plain aggregate (count(item.property)) succeeds. Oddly, the wrapped form does work inside a live query, so behavior is inconsistent.

Repro

Building the subquery outside of the hook threw the error:

const vehicleDispatchCountSubquery = new Query()
  .from({ vehicleDispatch: vehicleDispatchCollection })
  .groupBy(({ vehicleDispatch }) => vehicleDispatch.dispatchId)
  .select(({ vehicleDispatch }) => ({
    dispatchId: vehicleDispatch.dispatchId,
    vehicleDispatchCount: coalesce(count(vehicleDispatch.id), 0),
  }))

However, just using count(vehicleDispatch.id) without coalesce no longer throws the error.

Alternatively, within a useLiveQuery() nesting works without error:

export const useDispatchSidebarData = (date: string) =>
  useLiveQuery(
    q => {
      const vdCountSubquery = q
        .from({ vehicleDispatch: vehicleDispatchCollection })
        .groupBy(({ vehicleDispatch }) => vehicleDispatch.dispatchId)
        .select(({ vehicleDispatch }) => ({
          dispatchId: vehicleDispatch.dispatchId,
          vehicleDispatchCount: coalesce(count(vehicleDispatch.id)),
        }))
        .distinct()
      return q
        .from({ dispatch: dispatchCollection })
        .where(({ dispatch }) => eq(dispatch.date, date))
        .leftJoin({ vdCount: vdCountSubquery }, ({ dispatch, vdCount }) => eq(dispatch.id, vdCount.dispatchId))
        .select(({ dispatch, vdCount }) => ({
          ...dispatch,
          vehicleDispatchCount: vdCount?.vehicleDispatchCount,
        }))
    },
    [date]
  )

Error

QueryCompilationError: Unknown expression type: agg
    at compileExpressionInternal (…/chunk-TT3G3ME3.js:703:13)
    at …/chunk-TT3G3ME3.js:751:14
    at Array.map (<anonymous>)
    at compileFunction (…/chunk-TT3G3ME3.js:750:34)

Environment

Running in docker -> node:20-alpine -> node v20.19.5

{
    "@electric-sql/client": "^1.1.0",
    "@electric-sql/react": "^1.0.15",
    ...
    "@tanstack/electric-db-collection": "^0.1.38",
    "@tanstack/query-core": "^5.90.5",
    "@tanstack/query-db-collection": "^0.2.36",
    "@tanstack/react-db": "^0.1.36",
    ...
    "zod": "^4.1.12"
    "react": "^19.2.0",
    "react-dom": "^19.2.0",
    "vite": "^7.1.12",
    ...
    "babel-plugin-react-compiler": "^1.0.0",
}

Possible Cause

@samwillis in discord link

We may not yet support an aggregate expression wrapped by other expressions. Do open an issue, we should fix that.

[kevin-dp]

Hi @evanheckert,

I opened a PR #1274 that reproduces the problem. Note that the problem also occurs inside the useLiveQuery hook (contrary to what seemed to work in the original report). As @samwillis suspected, aggregates nested inside other expressions are currently not supported by the query compiler.