winnow/lib/query/unique-value-query.js

Apply sql-like filters to GeoJSON

const { create: createSqlStatement } = require('../sql-query-builder')
const normalizeQueryOptions = require('../normalize-query-options')
const standardQuery = require('./standard-query')

module.exports = function uniqueValueQuery (features, { fields } = {}) {
  if (fields.length > 3) throw new Error('Cannot classify using more than three fields')

  fields.map(field => {
    if (!features[0].properties[field]) throw new Error(`Unknown field: ${field}`)
  })

  const uniqueValueOptions = {
    aggregates: [
      {
        type: 'count',
        field: fields[0], // arbitrary field choice
        name: 'count'
      }
    ],
    groupBy: fields
  }

  const options = normalizeQueryOptions(uniqueValueOptions, features)
  const sql = createSqlStatement(options)
  return standardQuery(features, sql, { ...options, skipLimitHandling: true })
}