fix(model): handle passing string projection to hydrate()

lib/helpers/projection/parseProjection.js

@@ -1,8 +1,13 @@
 'use strict';
 
 /**
- * Convert a string or array into a projection object, retaining all
- * `-` and `+` paths.
+ * Convert a string or array into a projection object. Treats `-foo` as
+ * equivalent to `foo: 0` depending on `retainMinusPaths`. If `retainMinusPaths`
+ * is true, then `-foo` will be included in the projection as `'-foo': 0`.
+ *
+ * @param {object|string|string[]} v
+ * @param {boolean} [retainMinusPaths]
+ * @return {object}
  */
 
 module.exports = function parseProjection(v, retainMinusPaths) {
@@ -22,9 +27,9 @@ module.exports = function parseProjection(v, retainMinusPaths) {
     if (!field) {
       continue;
     }
-    const include = '-' == field[0] ? 0 : 1;
+    const include = field.charAt(0) === '-' ? 0 : 1;
     if (!retainMinusPaths && include === 0) {
-      field = field.substring(1);
+      field = field.slice(1);
     }
     ret[field] = include;
   }

lib/model.js

@@ -61,6 +61,7 @@ const decorateDiscriminatorIndexOptions = require('./helpers/indexes/decorateDis
 const isPathSelectedInclusive = require('./helpers/projection/isPathSelectedInclusive');
 const leanPopulateMap = require('./helpers/populate/leanPopulateMap');
 const parallelLimit = require('./helpers/parallelLimit');
+const parseProjection = require('./helpers/projection/parseProjection');
 const prepareDiscriminatorPipeline = require('./helpers/aggregate/prepareDiscriminatorPipeline');
 const pushNestedArrayPaths = require('./helpers/model/pushNestedArrayPaths');
 const removeDeselectedForeignField = require('./helpers/populate/removeDeselectedForeignField');
@@ -3935,7 +3936,7 @@ Model.buildBulkWriteOperations = function buildBulkWriteOperations(documents, op
  * @api public
  */
 
-Model.hydrate = function(obj, projection, options) {
+Model.hydrate = function hydrate(obj, projection, options) {
   _checkContext(this, 'hydrate');
 
   if (options?.virtuals && options?.hydratedPopulatedDocs === false) {
@@ -3946,6 +3947,7 @@ Model.hydrate = function(obj, projection, options) {
     if (obj?.$__ != null) {
       obj = obj.toObject(internalToObjectOptions);
     }
+    projection = parseProjection(projection);
     obj = applyProjection(obj, projection);
   }
   const document = require('./queryHelpers').createModel(this, obj, projection, projection, options);

test/model.hydrate.test.js

@@ -79,18 +79,33 @@ describe('model', function() {
       assert.deepEqual(['food'], Object.keys(err.errors));
     });
 
-    it('supports projection (gh-9209)', function() {
+    it('supports projection (gh-16082) (gh-9209)', function() {
       const schema = new Schema({
         prop: String,
         arr: [String]
       });
       const Model = db.model('Test2', schema);
 
-      const doc = Model.hydrate({ prop: 'test' }, { arr: 0 });
+      let doc = Model.hydrate({ prop: 'test' }, { arr: 0 });
+      assert.equal(doc.isNew, false);
+      assert.equal(doc.isModified(), false);
+      assert.ok(!doc.$__delta());
+      assert.strictEqual(doc.prop, 'test');
+      // Array implicit default of `[]` shouldn't apply
+      assert.strictEqual(doc.arr, undefined);
+
+      doc = Model.hydrate({ prop: 'test' }, '-arr');
+      assert.equal(doc.isNew, false);
+      assert.equal(doc.isModified(), false);
+      assert.ok(!doc.$__delta());
+      assert.strictEqual(doc.prop, 'test');
+      assert.strictEqual(doc.arr, undefined);
 
+      doc = Model.hydrate({ prop: 'test' }, ['_id']);
       assert.equal(doc.isNew, false);
       assert.equal(doc.isModified(), false);
       assert.ok(!doc.$__delta());
+      assert.ok(!doc.prop);
     });
 
     it('works correctly with model discriminators', function() {