René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Batch (and Transaction) dataset_id should match that of Key / Query objects when added","articleBody":"Proposed fix:\n\nDo not ask for `dataset_id` in the constructor and instead gather it from the keys and queries used in the transaction.\n\nThis may require implementing #495 and **will** require removing the `self.begin()` [call](https://github.com/GoogleCloudPlatform/gcloud-python/blob/8faba35a266cd6280f125adf6035f902a07a26e4/gcloud/datastore/transaction.py#L246) from `Transaction.__enter__`. (Can't begin until a query or entity is encountered.)\n\n**Title** was: \"Transaction does not rollback correctly when multiple datasets involved\"\n\n---\n\nWitness the following code:\n\n``` python\n\u003e\u003e\u003e from gcloud import datastore\n\u003e\u003e\u003e cnxn = datastore.get_connection()\n\u003e\u003e\u003e dataset1 = cnxn.dataset('PROJ1')\n\u003e\u003e\u003e dataset2 = cnxn.dataset('PROJ2')\n\u003e\u003e\u003e e1 = dataset1.entity('foo')\n\u003e\u003e\u003e e2 = dataset2.entity('foo')\n\u003e\u003e\u003e e1['bar'] = e2['bar'] = 0\n\u003e\u003e\u003e e1.save()\n\u003cEntity[{'kind': u'foo', 'id': 5686683802533888L}] {'bar': 0}\u003e\n\u003e\u003e\u003e e2.save()\n\u003cEntity[{'kind': u'foo', 'id': 5659313586569216L}] {'bar': 0}\u003e\n\u003e\u003e\u003e with dataset1.transaction():\n...     e1.delete()\n...     e2.delete()\n... \n\u003e\u003e\u003e e1_still = dataset1.get_entity(e1.key())\n\u003e\u003e\u003e e1_still is None\nTrue\n\u003e\u003e\u003e e2_still = dataset2.get_entity(e2.key())\n\u003e\u003e\u003e e2_still\n\u003cEntity[{'kind': u'foo', 'id': 5659313586569216L}] {u'bar': 0L}\u003e\n```\n","author":{"url":"https://github.com/dhermes","@type":"Person","name":"dhermes"},"datePublished":"2014-12-19T06:11:39.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":5},"url":"https://github.com/447/google-cloud-python/issues/447"}