UNWIND with js driver yields "Cannot merge node using null property ..."

[ekobi]

Hi,

Running into a brick wall using the javascript driver to migrate JSON data into Neo4j.

Simple transaction wrapper:

const n4jTxn = ({ neo4jDriver, unitsOfWork }) => {
  const session = neo4jDriver.session({ defaultAccessMode: neo4j.session.WRITE});
  return session.writeTransaction((txn) => {
    const promises = _.map(unitsOfWork, (uow) => {
      console.log(`[n4jTxn] unitOfWork: ${JSON.stringify(uow, {}, 2)}`);
      return txn.run({ text: uow.query, parameters: uow.parameters});
    });
    return Promise.all(promises);
  }).then((results) => {
    console.log(`[n4jTxn] result: ${JSON.stringify(results, {}, 2)}`);
    session.close();
    return results;
  }).catch((error) => {
    session.close();
    throw error;
  });

};

Yields the dreaded "Cannot merge node using null property value for xyz..."

[n4jTxn] unitOfWork: {
  "query": "UNWIND $users AS u MERGE (newUser:User {  _id: u._id, name: u.name, email: u.email, accountId: u.accountId }) RETURN newUser",
  "parameters": {
    "users": [
      {
        "_id": "5cc1efebe13133250e8bc9c6",
        "name": "Admin 1",
        "email": "admin1@example.com",
        "accountId": "de34d0a8-4715-40aa-8cc5-dasdfjad"
      },
      {
        "_id": "5cc1efebe13133250e8bc9c9",
        "name": "GC 40",
        "email": "gc40@examplecom",
        "accountId": "b70f5117-4f1f-45a9-8278-addkadk"
      },
      {
        "_id": "5cc1efebe13133250e8bc9cb",
        "name": "Architect 1",
        "email": "architect1@example.com",
        "accountId": "3937f941-040d-40ed-b1ee-alsdfa"
      }
    ]
  }
}
[migrate -- migrateUsers] Neo4jError: Cannot merge node using null property value for accountId.

Any thoughts on what's going on? Thanks.

[david_allen]

Specifically -- you don't have any xyz properties in there. It's not clear if you meant that as a generic example or not, because no xyz properties are mentioned in your code.

More generally -- MERGE is a combination of MATCH and CREATE. MERGE needs to first try to find nodes by a particular property set, so you can't use null property values with merge because of special details about how null operates. For example this won't work:

MERGE (f:Foo { x: null })

So the answer is to migrate any properties that might be null outside of the MERGE clause. Like this:

MERGE (f:Foo { id: 1 })
  SET f.otherProp = $param

This will work even if $param is null, where MERGE (f:Foo { id: 1, otherProp: $param }) would fail.

[ekobi]

david.allen:

Specifically -- you don't have any xyz properties in there. It's not clear if you meant that as a generic example or not, because no xyz properties are mentioned in your code.

The second code block shows the parameters getting passed on the wire, in addition to the query itself. I'm trying to apply this pattern to my data: Creating Nodes from a list parameter.

[ekobi · Accepted solution]

For the record, the issue seems to be that the js driver is not correctly serializing the parameters. Re-implementing the transaction wrapper to use the HTTP interface works as expected:

const urllib = require('urllib');
const txnURL= "http://localhost:7474/db/neo4j/tx/commit";
const n4jTxn = async (statements) => {
  //
  // expect statements to be an array of statement + query dicts:
  //
  //     statements: [{statement: query, parameters: params }] ;
  const data = { statements };
  // console.log(`[n4jTxn] payload data: ${JSON.stringify(data, {}, 2)}`);
  return urllib.request(txnURL, {dataType: 'json', contentType: 'json', auth: AUTH_STRING, method: 'POST', data })
    .catch((error) => {
      console.error(`[n4jTxn] error: ${JSON.stringify(error, {}, 2)}`);
      throw error;
    })
    .then((results) => {
      console.log(`[n4jTxn] results: ${JSON.stringify(results, {}, 2)}`);
      return results;
    });
};