fix: allow array getters to return non-arrays

index.js

@@ -125,11 +125,15 @@ function applyGettersToDoc(schema, doc, fields, prefix) {
     const pathExists = mpath.has(path, doc);
     if (pathExists) {
       if (schematype.$isMongooseArray && !schematype.$isMongooseDocumentArray) {
+        // A getter may return a non-array
+        const got = schematype.applyGetters(mpath.get(path, doc), doc, true);
+        const val = Array.isArray(got) ? got.map(subdoc => {
+          return schematype.caster.applyGetters(subdoc, doc);
+        }) : schematype.caster.applyGetters(got, doc);
+
         mpath.set(
           path,
-          schematype.applyGetters(mpath.get(path, doc), doc, true).map(subdoc => {
-            return schematype.caster.applyGetters(subdoc, doc);
-          }),
+          val,
           doc
         );
       } else {

test/index.test.js

@@ -384,4 +384,38 @@ describe('mongoose-lean-getters', function() {
     assert.equal(typeof doc.field, 'string');
     assert.strictEqual(doc.field, '1337');
   });
+
+  it('should should allow array getters to return non-arrays', async function() {
+    const userSchema = new mongoose.Schema({
+      emails: {
+        type: [String],
+        // returns undefined, string, or array
+        get: (val) => !val || !val.length ? undefined : (val.length === 1 ? val[0] : val),
+        set: (val) => typeof val === 'string' ? [val] : val,
+      }
+    });
+    userSchema.plugin(mongooseLeanGetters);
+    const User = mongoose.model('gh-37-transform-arrays', userSchema);
+
+    const variants = [
+      { sourceVal: 'foo', expectedVal: 'foo' },
+      { sourceVal: ['foo'], expectedVal: 'foo' },
+      { sourceVal: ['foo', 'bar'], expectedVal: ['foo', 'bar'] },
+      { sourceVal: [], expectedVal: undefined },
+      { sourceVal: null, expectedVal: undefined },
+      { sourceVal: undefined, expectedVal: undefined },
+    ];
+
+    await Promise.all(
+      variants.map(async({ sourceVal, expectedVal }) => {
+        const user = new User({ emails: sourceVal });
+        await user.save();
+
+        const foundUser = await User.findById(user._id).lean({ getters: true });
+        const stringified = JSON.stringify({ sourceVal, expectedVal });
+        assert.deepStrictEqual(user.emails, expectedVal, `user did not have expected value ${stringified}`);
+        assert.deepStrictEqual(foundUser.emails, expectedVal, `foundUser did not have expected value ${stringified}`);
+      })
+    );
+  });
 });