Advanced Index Definition

This guide describes how the indexes module let you create more complex types of indexes based on the data in Store objects.

Custom Sorting

As well as indicating what the Slice Ids in the Index should be, based on a Cell value in each Row, you can also indicate how the Row Ids will be sorted inside each Slice. For example, here the members of each species are sorted by weight, by specifying that CellId in the fourth parameter:

const store = createStore().setTable('pets', {
  fido: {species: 'dog', weight: 42},
  felix: {species: 'cat', weight: 13},
  cujo: {species: 'dog', weight: 37},
});

const indexes = createIndexes(store);
indexes.setIndexDefinition(
  'bySpecies', // indexId
  'pets', //      tableId to index
  'species', //   cellId to index
  'weight', //    cellId to sort by
);

console.log(indexes.getSliceRowIds('bySpecies', 'dog'));
// -> ['cujo', 'fido']

With a further fifth and sixth parameter, you can also indicate how the Slice Ids (and the Rows within them) should be sorted. These two parameters take a 'sorter' function (much like JavaScript's own array sort method) which compares pairs of values. For example, to order the species Slices alphabetically, and the animals in each species in reverse weight order:

indexes.setIndexDefinition(
  'bySpecies', // indexId
  'pets', //      tableId to index
  'species', //   cellId to index
  'weight', //    cellId to sort by
  (id1, id2) => (id1 < id2 ? -1 : 1), // Slices in alphabetical order
  (id1, id2) => (id1 > id2 ? -1 : 1), // Rows in reverse numerical order
);

console.log(indexes.getSliceIds('bySpecies'));
// -> ['cat', 'dog']
console.log(indexes.getSliceRowIds('bySpecies', 'dog'));
// -> ['fido', 'cujo']

Note that you can use the defaultSorter function from the common module (which is literally equivalent to (id1, id2) => (id1 < id2 ? -1 : 1)) if all you want to do is sort something alphanumerically.

Sorting is used in the Countries demo to sort both the Slice Ids (the first letters of the alphabet) and the Row Ids (the country names) within them.

Getting Custom Values From Rows

By default, our Index definitions have named a Cell in the Row which contains the string to use as the Slice Id - like the species Cell in the example above. Sometimes you may wish to derive a Slice Id for each Row that is not in a single Cell, and in this case you can replace the third parameter with a function which can process the Row in any way you wish.

For example, we could group our pets into 'heavy' and 'light' Slices, based on the range that the weight Cell lies in.:

indexes.setIndexDefinition(
  'byWeightRange', //                                           indexId
  'pets', //                                                    tableId to index
  (getCell) => (getCell('weight') > 40 ? 'heavy' : 'light'), // => sliceId
);

console.log(indexes.getSliceIds('byWeightRange'));
// -> ['heavy', 'light']
console.log(indexes.getSliceRowIds('byWeightRange', 'light'));
// -> ['felix', 'cujo']

You can also provide a function for the key to sort entries by. This sorts animal Row Ids, heaviest first, in each Slice:

indexes.setIndexDefinition(
  'byWeightRange', //                                           indexId
  'pets', //                                                    tableId to index
  (getCell) => (getCell('weight') > 40 ? 'heavy' : 'light'), // => sliceId
  (getCell) => -getCell('weight'), //                           => sort key
);

console.log(indexes.getSliceRowIds('byWeightRange', 'light'));
// -> ['cujo', 'felix']

And with that, we have covered most of the basics of using the indexes module. Let's move on to a very similar module for creating relationships between data - as well as tracking changes - in the Relationships And Checkpoints guide.