diff --git a/lib/model.js b/lib/model.js
index edfe7c5e61..e400536eb9 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -1302,19 +1302,21 @@ Model.syncIndexes = async function syncIndexes(options) {
     throw new MongooseError('Model.syncIndexes() no longer accepts a callback');
   }
 
-  const model = this;
+  const autoCreate = options?.autoCreate ?? this.schema.options?.autoCreate ?? this.db.config.autoCreate ?? true;
 
-  try {
-    await model.createCollection();
-  } catch (err) {
-    if (err != null && (err.name !== 'MongoServerError' || err.code !== 48)) {
-      throw err;
+  if (autoCreate) {
+    try {
+      await this.createCollection();
+    } catch (err) {
+      if (err != null && (err.name !== 'MongoServerError' || err.code !== 48)) {
+        throw err;
+      }
     }
   }
 
-  const diffIndexesResult = await model.diffIndexes();
-  const dropped = await model.cleanIndexes({ ...options, toDrop: diffIndexesResult.toDrop });
-  await model.createIndexes({ ...options, toCreate: diffIndexesResult.toCreate });
+  const diffIndexesResult = await this.diffIndexes();
+  const dropped = await this.cleanIndexes({ ...options, toDrop: diffIndexesResult.toDrop });
+  await this.createIndexes({ ...options, toCreate: diffIndexesResult.toCreate });
 
   return dropped;
 };
diff --git a/lib/schema/documentArray.js b/lib/schema/documentArray.js
index 9a7a5d3181..413dc4a8fb 100644
--- a/lib/schema/documentArray.js
+++ b/lib/schema/documentArray.js
@@ -522,6 +522,7 @@ SchemaDocumentArray.prototype.clone = function() {
   }
   schematype.Constructor.discriminators = Object.assign({},
     this.Constructor.discriminators);
+  schematype._appliedDiscriminators = this._appliedDiscriminators;
   return schematype;
 };
 
diff --git a/lib/schema/subdocument.js b/lib/schema/subdocument.js
index 5116ed0cfa..9a77d82c87 100644
--- a/lib/schema/subdocument.js
+++ b/lib/schema/subdocument.js
@@ -393,5 +393,6 @@ SchemaSubdocument.prototype.clone = function() {
     schematype.requiredValidator = this.requiredValidator;
   }
   schematype.caster.discriminators = Object.assign({}, this.caster.discriminators);
+  schematype._appliedDiscriminators = this._appliedDiscriminators;
   return schematype;
 };
diff --git a/test/model.test.js b/test/model.test.js
index 098624e2e9..e7bd52447a 100644
--- a/test/model.test.js
+++ b/test/model.test.js
@@ -4885,6 +4885,28 @@ describe('Model', function() {
         assert.deepStrictEqual(indexes.map(index => index.name), ['_id_', 'name_1']);
       });
 
+      it('avoids creating collection if autoCreate: false', async() => {
+        const collectionName = generateRandomCollectionName();
+        const userSchema = new Schema(
+          { name: { type: String, index: true } },
+          { autoIndex: false, autoCreate: false, collation: { locale: 'en_US', strength: 2 } }
+        );
+        const User = db.model('User', userSchema, collectionName);
+
+        // Act
+        await User.syncIndexes();
+
+        // Assert
+        const indexes = await User.listIndexes();
+        assert.deepStrictEqual(indexes.map(index => index.name), ['_id_', 'name_1']);
+
+        const collections = await User.db.listCollections();
+        const collection = collections.find(c => c.name === collectionName);
+        assert.ok(collection);
+        // Collation was not applied because autoCreate was false, so Mongoose did not send `createCollection()`
+        assert.ok(!collection.options.collation);
+      });
+
       it('drops indexes that are not present in schema', async() => {
         // Arrange
         const collectionName = generateRandomCollectionName();
@@ -8004,6 +8026,36 @@ describe('Model', function() {
     assert.equal(doc.items[0].prop, 42);
   });
 
+  it('does not throw with multiple self-referencing discriminator schemas applied to schema (gh-15120)', async function() {
+    const baseSchema = new Schema({
+      type: { type: Number, required: true }
+    }, { discriminatorKey: 'type' });
+
+    const selfRefSchema = new Schema({
+      self: { type: [baseSchema] }
+    });
+
+    const anotherSelfRefSchema = new Schema({
+      self2: { type: [baseSchema] }
+    });
+
+    baseSchema.discriminator(5, selfRefSchema);
+    baseSchema.discriminator(6, anotherSelfRefSchema);
+    const Test = db.model('Test', baseSchema);
+
+    const doc = await Test.create({
+      type: 5,
+      self: {
+        type: 6,
+        self2: null
+      }
+    });
+    assert.strictEqual(doc.type, 5);
+    assert.equal(doc.self.length, 1);
+    assert.strictEqual(doc.self[0].type, 6);
+    assert.strictEqual(doc.self[0].self2, null);
+  });
+
   it('inserts versionKey even if schema has `toObject.versionKey` set to false (gh-14344)', async function() {
     const schema = new mongoose.Schema(
       { name: String },