diff --git a/lib/helpers/discriminator/mergeDiscriminatorSchema.js b/lib/helpers/discriminator/mergeDiscriminatorSchema.js
index fcba6c231c4..b48d5c4db07 100644
--- a/lib/helpers/discriminator/mergeDiscriminatorSchema.js
+++ b/lib/helpers/discriminator/mergeDiscriminatorSchema.js
@@ -34,7 +34,8 @@ module.exports = function mergeDiscriminatorSchema(to, from, path, seen) {
         key === 'base' ||
         key === '_applyDiscriminators' ||
         key === '_userProvidedOptions' ||
-        key === 'options') {
+        key === 'options' ||
+        key === 'tree') {
         continue;
       }
     }
@@ -73,4 +74,8 @@ module.exports = function mergeDiscriminatorSchema(to, from, path, seen) {
       mergeDiscriminatorSchema(to[key], from[key], path ? path + '.' + key : key, seen);
     }
   }
+
+  if (from != null && from.instanceOfSchema) {
+    to.tree = Object.assign({}, from.tree, to.tree);
+  }
 };
diff --git a/test/model.discriminator.test.js b/test/model.discriminator.test.js
index 6f7fd05d4a2..400cf976f63 100644
--- a/test/model.discriminator.test.js
+++ b/test/model.discriminator.test.js
@@ -2180,4 +2180,27 @@ describe('model', function() {
     assert.ok(innerBuildingsPath.schemaOptions.type.discriminators.Garage);
     assert.equal(innerBuildingsPath.schemaOptions.type.discriminators.Garage.discriminatorMapping.value, 'G');
   });
+
+  it('runs base schema paths validators and setters before child schema validators and setters (gh-13794)', async function() {
+    const baseSchema = new Schema({
+      f1: {
+        type: Number,
+        set() {
+          return 1;
+        }
+      }
+    });
+    const childSchema = new Schema({
+      f2: {
+        type: Number,
+        set() {
+          return this.f1;
+        }
+      }
+    });
+    const Test = db.model('Test', baseSchema);
+    const Child = Test.discriminator('Child', childSchema);
+    const doc = new Child({ f1: 2, f2: 2 });
+    assert.strictEqual(doc.f2, 1);
+  });
 });