expression-helpers-queryfn.md

@tanstack/db	patch
@tanstack/query-db-collection	patch

Add expression helper utilities for parsing LoadSubsetOptions in queryFn.

When using syncMode: 'on-demand', TanStack DB now provides helper functions to easily parse where clauses, orderBy, and limit predicates into your API's format:

• parseWhereExpression: Parse where clauses with custom handlers for each operator
• parseOrderByExpression: Parse order by into simple array format
• extractSimpleComparisons: Extract simple AND-ed filters
• parseLoadSubsetOptions: Convenience function to parse all options at once
• walkExpression, extractFieldPath, extractValue: Lower-level helpers

Example:

import { parseLoadSubsetOptions } from "@tanstack/db"
// or from "@tanstack/query-db-collection" (re-exported for convenience)

queryFn: async (ctx) => {
  const { where, orderBy, limit } = ctx.meta.loadSubsetOptions

  const parsed = parseLoadSubsetOptions({ where, orderBy, limit })

  // Build API request from parsed filters
  const params = new URLSearchParams()
  parsed.filters.forEach(({ field, operator, value }) => {
    if (operator === "eq") {
      params.set(field.join("."), String(value))
    }
  })

  return fetch(`/api/products?${params}`).then((r) => r.json())
}

This eliminates the need to manually traverse expression AST trees when implementing predicate push-down.