demo.js

const dgraph = require('dgraph-js');

const clientStubs = [
	new dgraph.DgraphClientStub('127.0.0.1:9080'),
];

const client = new dgraph.DgraphClient(...clientStubs);

const cleanup = async () => {
	const txn = client.newTxn();

	try {
		const req = new dgraph.Request();

		req.setQuery(`
			query {
				bars as var(func: type(Bar))
				foos as var(func: type(Foo))
			}
		`);

		const mu1 = new dgraph.Mutation();

		mu1.setDeleteJson({
			uid: 'uid(bars)',
		});

		req.addMutations(mu1);

		const mu2 = new dgraph.Mutation();

		mu2.setDeleteJson({
			uid: 'uid(foos)',
		});

		req.addMutations(mu2);

		await txn.doRequest(req);
		await txn.commit();
	} finally {
		await txn.discard();
	}
};

const initBar = async () => {
	const txn = client.newTxn();

	try {
		const req = new dgraph.Request();

		['bar 1', 'bar 2', 'bar 3'].forEach(name => {
			const mu = new dgraph.Mutation();

			mu.setSetJson({
				bar_name: name,
				'dgraph.type': 'Bar',
			});

			req.addMutations(mu);
		});

		await txn.doRequest(req);
		await txn.commit();
	} finally {
		await txn.discard();
	}
};

const initFoo = async () => {
	const txn = client.newTxn();

	try {
		const req = new dgraph.Request();

		req.setQuery(`
			query {
				bar2 as var(func: eq(bar_name, "bar 2"))
			}
		`);

		const mu = new dgraph.Mutation();

		mu.setSetJson({
			'bar': 'uid(bar2)',
			'dgraph.type': 'Foo',
		});

		req.addMutations(mu);

		await txn.doRequest(req);
		await txn.commit();
	} catch (err) {
		console.error(err);
	} finally {
		await txn.discard();
	}
};

const preCommit = async () => {
	const txn = client.newTxn();

	try {
		const req = new dgraph.Request();

		// Switch to bar 3
		req.setQuery(`
			query {
				foo as var(func: type(Foo))
				bar3 as var(func: eq(bar_name, "bar 3"))
			}
		`);

		const mu = new dgraph.Mutation();

		mu.setSetJson({
			bar: 'uid(bar3)',
			'dgraph.type': 'Foo',
			uid: 'uid(foo)',
		});

		req.addMutations(mu);

		await txn.doRequest(req);

		const result = await txn.queryWithVars(`
			query {
				foo(func: type(Foo)) {
					uid
					bar {
						bar_name
					}
				}
			}
		`);

		console.log('Pre-commit');
		console.log(JSON.stringify(result.getJson(), null, 2));

		await txn.commit();
	} catch (err) {
		console.error(err);
	} finally {
		await txn.discard();
	}
};

const postCommit = async () => {
	const txn = client.newTxn();

	try {
		const result = await txn.queryWithVars(`
			query {
				foo(func: type(Foo)) {
					uid
					bar {
						bar_name
					}
				}
			}
		`);

		console.log('Post-commit');
		console.log(JSON.stringify(result.getJson(), null, 2));
	} catch (err) {
		console.error(err);
	} finally {
		await txn.discard();
	}
};

const doItAll = async () => {
	await cleanup();
	await initBar();
	await initFoo();
	await preCommit();
	await postCommit();
};

doItAll();

output.txt

# Ran tmp.js repeatedly to show what's going on:
bash-3.2$ node tmp.js
Pre-commit
{
  "foo": [
    {
      "uid": "0x1fd",
      "bar": {
        "bar_name": "bar 3"
      }
    }
  ]
}
Post-commit
{
  "foo": [
    {
      "uid": "0x1fd",
      "bar": {
        "bar_name": "bar 3"
      }
    }
  ]
}
bash-3.2$ node tmp.js
Pre-commit
{
  "foo": [
    {
      "uid": "0x201",
      "bar": {
        "bar_name": [
          "bar 2",
          "bar 3"
        ]
      }
    }
  ]
}
Post-commit
{
  "foo": [
    {
      "uid": "0x201",
      "bar": {
        "bar_name": "bar 3"
      }
    }
  ]
}