feat(populate): introduce populate ordered option

Author: vkarpov15

lib/document.js

@@ -4525,15 +4525,21 @@ Document.prototype.populate = async function populate() {
     throw new MongooseError('Document.prototype.populate() no longer accepts a callback');
   }
 
+  let ordered = null;
   if (args.length !== 0) {
     // use hash to remove duplicate paths
     const res = utils.populate.apply(null, args);
+    ordered = res.ordered;
     for (const populateOptions of res) {
       pop[populateOptions.path] = populateOptions;
     }
   }
 
   const paths = utils.object.vals(pop);
+  if (ordered) {
+    paths.ordered = ordered;
+  }
+
   let topLevelModel = this.constructor;
   if (this.$__isNested) {
     topLevelModel = this.$__[scopeSymbol].constructor;

lib/model.js

@@ -4275,11 +4275,20 @@ Model.populate = async function populate(docs, paths) {
   }
 
   // each path has its own query options and must be executed separately
-  const promises = [];
-  for (const path of paths) {
-    promises.push(_populatePath(this, docs, path));
+  if (paths.ordered) {
+    // Populate in series, primarily for transactions because MongoDB doesn't support multiple operations on
+    // one transaction in parallel.
+    for (const path of paths) {
+      await _populatePath(this, docs, path);
+    }
+  } else {
+    // By default, populate in parallel
+    const promises = [];
+    for (const path of paths) {
+      promises.push(_populatePath(this, docs, path));
+    }
+    await Promise.all(promises);
   }
-  await Promise.all(promises);
 
   return docs;
 };
@@ -4399,12 +4408,22 @@ async function _populatePath(model, docs, populateOptions) {
     return;
   }
 
-  const promises = [];
-  for (const arr of params) {
-    promises.push(_execPopulateQuery.apply(null, arr).then(valsFromDb => { vals = vals.concat(valsFromDb); }));
+  if (populateOptions.ordered) {
+    // Populate in series, primarily for transactions because MongoDB doesn't support multiple operations on
+    // one transaction in parallel.
+    for (const arr of params) {
+      await _execPopulateQuery.apply(null, arr).then(valsFromDb => { vals = vals.concat(valsFromDb); });
+    }
+  } else {
+    // By default, populate in parallel
+    const promises = [];
+    for (const arr of params) {
+      promises.push(_execPopulateQuery.apply(null, arr).then(valsFromDb => { vals = vals.concat(valsFromDb); }));
+    }
+
+    await Promise.all(promises);
   }
 
-  await Promise.all(promises);
 
   for (const arr of params) {
     const mod = arr[0];

lib/utils.js

@@ -523,7 +523,11 @@ exports.populate = function populate(path, select, model, match, options, subPop
 
     if (Array.isArray(path)) {
       const singles = makeSingles(path);
-      return singles.map(o => exports.populate(o)[0]);
+      const ret = singles.map(o => exports.populate(o)[0]);
+      if (path.ordered) {
+        ret.ordered = path.ordered;
+      }
+      return ret;
     }
 
     if (exports.isObject(path)) {
@@ -551,8 +555,8 @@ exports.populate = function populate(path, select, model, match, options, subPop
     };
   }
 
-  if (typeof obj.path !== 'string') {
-    throw new TypeError('utils.populate: invalid path. Expected string. Got typeof `' + typeof path + '`');
+  if (typeof obj.path !== 'string' && !(Array.isArray(obj.path) && obj.path.every(el => typeof el === 'string'))) {
+    throw new TypeError('utils.populate: invalid path. Expected string or array of strings. Got typeof `' + typeof path + '`');
   }
 
   return _populateObj(obj);
@@ -600,7 +604,11 @@ function _populateObj(obj) {
   }
 
   const ret = [];
-  const paths = oneSpaceRE.test(obj.path) ? obj.path.split(manySpaceRE) : [obj.path];
+  const paths = oneSpaceRE.test(obj.path)
+    ? obj.path.split(manySpaceRE)
+    : Array.isArray(obj.path)
+      ? obj.path
+      : [obj.path];
   if (obj.options != null) {
     obj.options = clone(obj.options);
   }
@@ -609,6 +617,13 @@ function _populateObj(obj) {
     ret.push(new PopulateOptions(Object.assign({}, obj, { path: path })));
   }
 
+  if (obj.ordered) {
+    ret.ordered = true;
+    for (const path of ret) {
+      path.ordered = true;
+    }
+  }
+
   return ret;
 }