feat: add beginTransaction API on datasource

add beginTransaction method which calls begin
method from the Transaction class which in turn
calls the connector's beginTransaction method if
it supports transactions.

Co-Authored-By: Miroslav Bajtoš <mbajtoss@gmail.com>

lib/datasource.js

@@ -2648,6 +2648,17 @@ DataSource.prototype.execute = function(command, args = [], options = {}) {
   }
 };
 
+/**
+ * Begin a new Transaction.
+ *
+ *
+ * @param [options] Options {isolationLevel: '...', timeout: 1000}
+ * @returns Promise A promise which resolves to a Transaction object
+ */
+DataSource.prototype.beginTransaction = function(options) {
+  return Transaction.begin(this.connector, options);
+};
+
 /*! The hidden property call is too expensive so it is not used that much
 */
 /**

test/transaction.test.js

@@ -71,6 +71,13 @@ describe('Transactions on test connector without execute()', () => {
     }, done);
   });
 
+  it('beginTransaction returns a transaction', async () => {
+    const promise = db.beginTransaction(Transaction.READ_UNCOMMITTED);
+    promise.should.be.Promise();
+    const transaction = await promise;
+    transaction.should.be.instanceof(EventEmitter);
+  });
+
   it('exposes and caches slave models', done => {
     testModelCaching(tx.models, db.models);
     done();

types/datasource.d.ts

@@ -12,6 +12,7 @@ import {
   PropertyDefinition,
 } from './model';
 import {EventEmitter} from 'events';
+import {IsolationLevel, Transaction} from './transaction-mixin';
 
 /**
  * LoopBack models can manipulate data via the DataSource object.
@@ -185,4 +186,15 @@ export declare class DataSource extends EventEmitter {
     args?: any[] | object,
     options?: Options
   ): Promise<any>;
+
+/**
+ * Begin a new transaction.
+ *
+ *
+ * @param [options] Options {isolationLevel: '...', timeout: 1000}
+ * @returns Promise A promise which resolves to a Transaction object
+ */
+  beginTransaction(
+    options?: IsolationLevel | Options,
+  ): PromiseOrVoid<Transaction>;
 }