Allow the use of database offsets when getting connections about relay

[solher]

I'm going to work on it ;)

[solher]

There is an issue as a lot of databases doesn't support the backward pagination.

The simplest solution would be to sort the results backward before applying the limit/offset.
We would maybe need a third return boolean parameter reverse.
Or maybe use negative offset and limit values.

[sogko]

hi @solher thanks for taking your time to look into this project, appreciate it very much 👍🏻

If the client asks for the first two elements, we potentially query 10000 documents from the database before throwing away the 9998 we don't want.

So the example solution you gave seems about right and it would address your issue, except that current relay-go library is missing a couple of pieces that would help to make it work

• relay.computeOffset: Actually there is already relay.getOffset().
  It translates a connection cursor string to an offset value that you can use to construct your query for your data source (SQL, Mongo etc)
  This is currently a private method. This should be exposed and make it available.
• relay.ConnectionFromArray() assume that the input data is static or representative of the whole data collection. Thats why currently your implementation might have to get every data for it to return correct cursor Connection value. And you are right to think that this is not ideal.

The missing piece is actually relay.ConnectionFromSliceArray() introduced a couple of months back in relay-js.
This has not been ported to relay-go

{ConnectionFromSliceArray} effectively allows you to fetch the data for a subset of a connection, and pass that in with some metadata indicating where the subsequence starts and how long the total array would be if materialized, and get a connection object back.

I can't personally commit when these changes would be made available but PRs are greatly welcomed!

Hope this answers your question!

TODO:

• Expose relay.GetOffset()
• Port and expose relay.ConnectionFromSliceArray()
• Secondary todo: Port latest changes from relay-js to relay-go (From 0.3.2 to 0.3.6)

[sogko]

Merged PR #22
Thanks @solher 👍🏻