fix(populate): handle multiple localFields + foreignFields using `loc…

…alField: function() {}` syntax

Fix #5704

lib/model.js

@@ -3620,8 +3620,18 @@ function populate(model, docs, options, callback) {
     }
 
     hasOne = true;
-    if (mod.foreignField !== '_id' || !match['_id']) {
-      match[mod.foreignField] = { $in: ids };
+    if (mod.foreignField.size === 1) {
+      const foreignField = Array.from(mod.foreignField)[0];
+      if (foreignField !== '_id' || !match['_id']) {
+        match[foreignField] = { $in: ids };
+      }
+    } else {
+      match.$or = [];
+      for (const foreignField of mod.foreignField) {
+        if (foreignField !== '_id' || !match['_id']) {
+          match.$or.push({ [foreignField]: { $in: ids } });
+        }
+      }
     }
 
     const assignmentOpts = {};
@@ -3653,9 +3663,11 @@ function populate(model, docs, options, callback) {
     // field is not selected, automatically select it because mongoose needs it.
     // If projection is exclusive and client explicitly unselected the foreign
     // field, that's the client's fault.
-    if (mod.foreignField !== '_id' && query.selectedInclusively() &&
-      !isPathSelectedInclusive(query._fields, mod.foreignField)) {
-      query.select(mod.foreignField);
+    for (const foreignField of mod.foreignField) {
+      if (foreignField !== '_id' && query.selectedInclusively() &&
+          !isPathSelectedInclusive(query._fields, foreignField)) {
+        query.select(foreignField);
+      }
     }
 
     // If we need to sub-populate, call populate recursively
@@ -3707,8 +3719,11 @@ function populate(model, docs, options, callback) {
     // the query result.
     for (let i = 0; i < len; i++) {
       val = vals[i];
-      if (val) {
-        _val = utils.getValue(mod.foreignField, val);
+      if (val == null) {
+        continue;
+      }
+      for (const foreignField of mod.foreignField) {
+        _val = utils.getValue(foreignField, val);
         if (Array.isArray(_val)) {
           const _valLength = _val.length;
           for (let j = 0; j < _valLength; ++j) {
@@ -3767,7 +3782,6 @@ function populate(model, docs, options, callback) {
       // If virtual, make sure to not mutate original field
       rawIds: mod.isVirtual ? allIds : mod.allIds,
       allIds: allIds,
-      localField: mod.localField,
       foreignField: mod.foreignField,
       rawDocs: rawDocs,
       rawOrder: rawOrder,
@@ -4060,15 +4074,16 @@ function getModelsMapForPopulate(model, docs, options) {
           docs: [doc],
           ids: [ids],
           allIds: [ret],
-          // Assume only 1 localField + foreignField
-          localField: localField,
-          foreignField: foreignField,
+          localField: new Set([localField]),
+          foreignField: new Set([foreignField]),
           justOne: justOne,
           isVirtual: isVirtual,
           virtual: virtual
         };
         map.push(available[modelName]);
       } else {
+        available[modelName].localField.add(localField);
+        available[modelName].foreignField.add(foreignField);
         available[modelName].docs.push(doc);
         available[modelName].ids.push(ids);
         available[modelName].allIds.push(ret);