Array fields

When a field in a document is an array, NeDB first tries to see if the query value is an array to perform an exact match, then whether there is an array-specific comparison function (for now there is only $size and $elemMatch) being used. If not, the query is treated as a query on every element and there is a match if at least one element matches.

  • $size: match on the size of the array
  • $elemMatch: matches if at least one array element matches the query entirely
  1. // Exact match
  2. db.find({ satellites: ['Phobos', 'Deimos'] }, function (err, docs) {
  3.   // docs contains Mars
  4. })
  5. db.find({ satellites: ['Deimos', 'Phobos'] }, function (err, docs) {
  6.   // docs is empty
  7. })
  9. // Using an array-specific comparison function
  10. // $elemMatch operator will provide match for a document, if an element from the array field satisfies all the conditions specified with the `$elemMatch` operator
  11. db.find({ completeData: { planets: { $elemMatch: { name: 'Earth', number: 3 } } } }, function (err, docs) {
  12.   // docs contains documents with id 5 (completeData)
  13. });
  15. db.find({ completeData: { planets: { $elemMatch: { name: 'Earth', number: 5 } } } }, function (err, docs) {
  16.   // docs is empty
  17. });
  19. // You can use inside #elemMatch query any known document query operator
  20. db.find({ completeData: { planets: { $elemMatch: { name: 'Earth', number: { $gt: 2 } } } } }, function (err, docs) {
  21.   // docs contains documents with id 5 (completeData)
  22. });
  24. // Note: you can't use nested comparison functions, e.g. { $size: { $lt: 5 } } will throw an error
  25. db.find({ satellites: { $size: 2 } }, function (err, docs) {
  26.   // docs contains Mars
  27. });
  29. db.find({ satellites: { $size: 1 } }, function (err, docs) {
  30.   // docs is empty
  31. });
  33. // If a document's field is an array, matching it means matching any element of the array
  34. db.find({ satellites: 'Phobos' }, function (err, docs) {
  35.   // docs contains Mars. Result would have been the same if query had been { satellites: 'Deimos' }
  36. });
  38. // This also works for queries that use comparison operators
  39. db.find({ satellites: { $lt: 'Amos' } }, function (err, docs) {
  40.   // docs is empty since Phobos and Deimos are after Amos in lexicographical order
  41. });
  43. // This also works with the $in and $nin operator
  44. db.find({ satellites: { $in: ['Moon', 'Deimos'] } }, function (err, docs) {
  45.   // docs contains Mars (the Earth document is not complete!)
  46. });