feat(settings): allow keeping index settings using mergeSettings (#153)

* adding merge settings option

* adding docs

Author: c4ndybar

README.md

@@ -69,6 +69,7 @@ const queries = [
       // Note: by supplying settings, you will overwrite all existing settings on the index
     },
     matchFields: ['slug', 'modified'], // Array<String> overrides main match fields, optional
+    mergeSettings: false, // optional, defaults to false.  See notes on mergeSettings below
   },
 ];
 
@@ -131,6 +132,14 @@ You can also specify `matchFields` per query to check for different fields based
 
 You can set settings for each index individually (per query), or otherwise it will keep your existing settings.
 
+### Merge Settings
+
+`mergeSettings` allows you to preserve settings changes made on the Algolia website.  The default behavior (`mergeSettings: false`) will wipe out your index settings and replace them with settings from the config on each build.
+
+When set to true, the config index settings will be merged with the existing index settings in Algolia (with the config index settings taking precendence).
+
+NOTE: When using `mergeSettings`, any **deleted** settings from the config settings will continue to be persisted since they will still exist in Algolia. If you want to remove a setting, be sure to remove it from both the config and on Algolia's website.
+
 ### Replicas
 
 For replica settings, extra care is taken to make sure only apply replicas to non-temporary indices.

gatsby-node.js

@@ -318,10 +318,11 @@ async function runIndexQueries(
 
   // defer to first query for index settings
   // todo: maybe iterate over all settings and throw if they differ
-  const { settings = mainSettings, forwardToReplicas } = queries[0] || {};
+  const { settings = mainSettings, mergeSettings = false, forwardToReplicas } = queries[0] || {};
 
   const settingsToApply = await getSettingsToApply({
     settings,
+    mergeSettings,
     index,
     tempIndex,
     indexToUse,
@@ -399,6 +400,7 @@ async function getIndexToUse({ index, tempIndex, enablePartialUpdates }) {
 /**
  * @param {object} options
  * @param {import('@algolia/client-search').Settings} options.settings
+ * @param {boolean} query.mergeSettings
  * @param {import('algoliasearch').SearchIndex} options.index
  * @param {import('algoliasearch').SearchIndex} options.tempIndex
  * @param {import('algoliasearch').SearchIndex} options.indexToUse
@@ -407,6 +409,7 @@ async function getIndexToUse({ index, tempIndex, enablePartialUpdates }) {
  */
 async function getSettingsToApply({
   settings,
+  mergeSettings,
   index,
   tempIndex,
   indexToUse,
@@ -427,6 +430,7 @@ async function getSettingsToApply({
   );
 
   const { replicaUpdateMode, ...requestedSettings } = {
+    ...(mergeSettings ? existingSettings : {}),
     ...settings,
     replicas: replicasToSet,
   };