some progress

node_modules/mongoose/lib/helpers/indexes/isIndexEqual.js

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+'use strict';
+
+const get = require('../get');
+const utils = require('../../utils');
+/**
+ * Given a Mongoose index definition (key + options objects) and a MongoDB server
+ * index definition, determine if the two indexes are equal.
+ *
+ * @param {Object} schemaIndexKeysObject the Mongoose index spec
+ * @param {Object} options the Mongoose index definition's options
+ * @param {Object} dbIndex the index in MongoDB as returned by `listIndexes()`
+ * @api private
+ */
+
+module.exports = function isIndexEqual(schemaIndexKeysObject, options, dbIndex) {
+  // Special case: text indexes have a special format in the db. For example,
+  // `{ name: 'text' }` becomes:
+  // {
+  //   v: 2,
+  //   key: { _fts: 'text', _ftsx: 1 },
+  //   name: 'name_text',
+  //   ns: 'test.tests',
+  //   background: true,
+  //   weights: { name: 1 },
+  //   default_language: 'english',
+  //   language_override: 'language',
+  //   textIndexVersion: 3
+  // }
+  if (dbIndex.textIndexVersion != null) {
+    delete dbIndex.key._fts;
+    delete dbIndex.key._ftsx;
+    const weights = { ...dbIndex.weights, ...dbIndex.key };
+    if (Object.keys(weights).length !== Object.keys(schemaIndexKeysObject).length) {
+      return false;
+    }
+    for (const prop of Object.keys(weights)) {
+      if (!(prop in schemaIndexKeysObject)) {
+        return false;
+      }
+      const weight = weights[prop];
+      if (weight !== get(options, 'weights.' + prop) && !(weight === 1 && get(options, 'weights.' + prop) == null)) {
+        return false;
+      }
+    }
+
+    if (options['default_language'] !== dbIndex['default_language']) {
+      return dbIndex['default_language'] === 'english' && options['default_language'] == null;
+    }
+
+    return true;
+  }
+
+  const optionKeys = [
+    'unique',
+    'partialFilterExpression',
+    'sparse',
+    'expireAfterSeconds',
+    'collation'
+  ];
+  for (const key of optionKeys) {
+    if (!(key in options) && !(key in dbIndex)) {
+      continue;
+    }
+    if (key === 'collation') {
+      if (options[key] == null || dbIndex[key] == null) {
+        return options[key] == null && dbIndex[key] == null;
+      }
+      const definedKeys = Object.keys(options.collation);
+      const schemaCollation = options.collation;
+      const dbCollation = dbIndex.collation;
+      for (const opt of definedKeys) {
+        if (get(schemaCollation, opt) !== get(dbCollation, opt)) {
+          return false;
+        }
+      }
+    } else if (!utils.deepEqual(options[key], dbIndex[key])) {
+      return false;
+    }
+  }
+
+  const schemaIndexKeys = Object.keys(schemaIndexKeysObject);
+  const dbIndexKeys = Object.keys(dbIndex.key);
+  if (schemaIndexKeys.length !== dbIndexKeys.length) {
+    return false;
+  }
+  for (let i = 0; i < schemaIndexKeys.length; ++i) {
+    if (schemaIndexKeys[i] !== dbIndexKeys[i]) {
+      return false;
+    }
+    if (!utils.deepEqual(schemaIndexKeysObject[schemaIndexKeys[i]], dbIndex.key[dbIndexKeys[i]])) {
+      return false;
+    }
+  }
+
+  return true;
+};