Support updating the primary key of an index to nil

[macraig]

Moved from meilisearch/meilisearch-go#449 by https://github.com/Mandrenkov

Description
To my best knowledge, it is not currently possible to update the primary key of an index to nil. Sure, you can create an index with a nil primary key, but if you change the primary key to some string, there is no way to update it back to nil.

Basic example

Ideally, it would be possible to write either

index.UpdateIndex("")

or

index.UpdateIndex(nil)

to update the primary key of index to nil.

Other

I'm no expert in Go or Meilisearch, but it looks like this might be the case because

type UpdateIndexRequest struct {
	PrimaryKey string `json:"primaryKey"`
}

is not annotated with omitempty as in

type UpdateIndexRequest struct {
	PrimaryKey string `json:"primaryKey,omitempty"`
}

and thus the server always receives "" instead of null.

For some context, I'm looking into developing a Terraform provider for Meilisearch in order to manage Meilisearch indices using configuration files (as opposed to manual API calls or scripts). For example, I can configure a Meilisearch index using code which looks like this:

resource "meilisearch_index" "foo" {
    uid         = "foo"
    primary_key = "bar"

    settings = {
        stop_words = ["some", "stop", "words"]
    }
}

To do so, I'm using the Meilisearch Go library to perform CRUD operations on Meilisearch indices. For example, if I define a Meilisearch index using

resource "meilisearch_index" "foo" {
    uid = "foo"
}

I can create the index using CreateIndex(&IndexConfig{Uid: "foo"}) and the primary key is set to null as expected. If I then decide to set another primary key as in

resource "meilisearch_index" "foo" {
    uid         = "foo"
    primary_key = "bar"
}

I can simply call UpdateIndex("bar") and the primary key is set to bar. So far, so good! But now, if I change my mind and decide I'm not sure what the primary key should be and go back to

resource "meilisearch_index" "foo" {
    uid = "foo"
}

there is no way, using the Meilisearch Go library, to set the primary key back to null!

Specifically, what happens when you try to update the primary key? Do you receive an error message?

Well, the issue is that there is no way to update the primary key to null. If I try to use something like

index.UpdateIndex("")

the primary key is set to "" (instead of null). Of course, trying to call

index.UpdateIndex(nil)

directly results in a type error as string values cannot be nil.

Also, do you have documents in the index? What is your use case?

I don't have any documents in the index. My use case is configuring the index before uploading documents to it.

---

Perhaps it would help if I phrased this issue another way. Suppose that you have an index, with UID foo, with a primary key set to bar. After looking into the requirements some more, you realize you can't predict the primary key ahead of time and would like Meilisearch to guess it. Unfortunately, there is no way you can update the primary key to be nil now and so your only course of action is to delete index foo and then recreate it with a null primary key!