Allow nested attr paths (dot notation) for exists conditions

[rbdmorgan]

Fixes #214 by allowing nested attr paths (dot notation) for exists conditions.

e.g. currently this:

{
  conditions: {
    attr: 'a.b.c',
    exists: true
  }
}

Generates this (which doesn't work):

{
  ExpressionAttributeNames: {
    '#attr1': 'a.b.c'
  },
  ConditionExpression: 'attribute_exists(#attr_1)'
}

With this PR, it will instead generate this (which does work):

{
  ExpressionAttributeNames: {
    '#attr1_0': 'a',
    '#attr1_1': 'b',
    '#attr1_2': 'c'
  },
  ConditionExpression: 'attribute_exists(#attr_1_0.#attr_1_1.#attr_1_2)'
}

The AWS docs suggest that attribute_exists(a.b.c) should work but it doesn't, at least not via the JS SDK. I found out about the above syntax and workaround on Stack Overflow!

Although I only need this fixed for the exists conditions, for completeness it looks like I probably should implement this for the between, in, begins_with, contains, attribute_type and size conditions/operators as well.

Keen to hear thoughts!

Just to add - I considered using arrays for nested attr paths, e.g.

{
  conditions: {
    attr: ['a', 'b', 'c'],
    exists: true
  }
}

As technically, you could have a valid (non-map) attribute key that is 'a.b.c' and want to check if that exists.

However, I don't think many people use that syntax, and it seems dot notation is assumed as the convention for nested attributes in checkAttribute(), and I didn't think it made sense to introduce a different convention here.

[jeremydaly]

Thanks @rbdmorgan! This is great. I'll give it a look.

[rbdmorgan]

@jeremydaly anything I can do to help move this along?

I'm happy to implement this for the other operators as well (between, in etc) but didn't want to go ahead unless you're happy with the approach I've taken.

Hoping to get this merged soon as I need it for a project I'm working on (currently I have these conditions commented out!)

Very much appreciated 😄

[jeremydaly]

HI @rbdmorgan, got a little overwhelmed last week. I'll review this today and try to get it merged! Then if you can extend to the other operators, that'd be great!

[rbdmorgan]

@jeremydaly that would be immensely appreciated if you get time!

And then I'll extend to the other operators over the next day or two 👍🏼