Support for custom Update expressions

[lsglick]

Occasionally we would like to use Update expressions for more advanced updates than seem to be available when modifying an attribute and using the existing #update methods. We particularly need this for transactions.

For example:

• Use ADD/DELETE to modify items in a set
• Use if_not_exists function to control behavior when multiple updates occur in succession
• Accumulate a value during a transaction (similar to increment! but in a transaction)

It would be great if there were a way to do this in special cases while still largely taking advantages of the conveniences afforded by this SDK.

Unfortunately, update_expression is always set directly with no opportunity to override it.

E.g. Item Operations:
https://github.com/aws/aws-sdk-ruby-record/blob/92e0070eea16698e23be8e31d3a36e681dc079e9/lib/aws-record/record/item_operations.rb#L609

E.g. Transactions:
https://github.com/aws/aws-sdk-ruby-record/blob/92e0070eea16698e23be8e31d3a36e681dc079e9/lib/aws-record/record/transactions.rb#L294

Some potential ideas:

• If update_expression is passed in opts, use it instead of a generated one.
• If update_expression is passed in opts, merge it with the generated one (might need to combine with the generated SET/REMOVE clauses)
• A way to pass-through raw update clauses to Aws::Record::Transactions.transact_write that only set the minimal key/table rather than the full transformation (similar to delete).
• A way to generate more raw actions similar to #transact_check_expression.

Thanks!

[mullermp]

Thanks for opening an issue. Tagging as a feature request. I am happy to review any pull requests if you have bandwidth. I recently plumbed through some options to save and update. I think option 1 (using opts, generally, over default values) is reasonable and I'd like to see that throughout item_operations and transactions usage.

[lsglick]

@mullermp I took a pass at it in #138. Let me know what you think!

There's a bit of logic duplication, but I was mostly trying to following existing structure in the repo.