Using same field on update expression and where condition causes update value to be overwritten

[onhate]

In an operation where you have the same field on update clause and where clause, the value on the where clause overwrites the values from the update.

await Table.update({id: '5'}, {$set: {foo: null}}).where({foo: 'bar'}).exec();

I would expect the query to be something like this:

{
        TableName: 'Table',
        ReturnValues: 'ALL_NEW',
        Key: {
            id: '5'
        },
        UpdateExpression: 'SET #k_foo=:v_foo',
        ExpressionAttributeNames: {
            '#k_foo': 'foo',
            '#k_id': 'id'
        },
        ExpressionAttributeValues: {
            ':v_foo': null,
            ':v_foo_1': 'bar',
            ':v_id': '5'
        },
        ConditionExpression: '(#k_id=:v_id) AND (#k_foo=:v_foo_1)'
    }

but it comes as this, and instead of setting the foo value to null it sets it back to bar.

{
        TableName: 'Table',
        ReturnValues: 'ALL_NEW',
        Key: {
            id: '5'
        },
        UpdateExpression: 'SET #k_foo=:v_foo',
        ExpressionAttributeNames: {
            '#k_foo': 'foo',
            '#k_id': 'id'
        },
        ExpressionAttributeValues: {
            ':v_foo': 'bar',
            ':v_id': '5'
        },
        ConditionExpression: '(#k_id=:v_id) AND (#k_foo=:v_foo)'
    }