Provide informative error message in case of unknown suggestion context.

[nilabhsagar]

At present providing unknown context in a suggestion context query does not provide informative error message.
Consider below mapping and correspongding query

Mapping

{
    "mappings": {
        "products" : {
            "properties" : {
                "suggest" : {
                    "type" : "completion",
                    "contexts": [
                        { 
                            "name": "product_type",
                            "type": "category",
                            "path": "cat"
                        },
                        { 
                            "name": "product_color",
                            "type": "category",
                            "path": "cat"
                        },
                        { 
                            "name": "product_size",
                            "type": "category",
                            "path": "cat"
                        }
                    ]
                }
            }
        }
    }
}

Query

{
    "suggest": {
        "products_suggestion" : {
            "prefix" : "sho",
            "completion" : {
                "field" : "suggest",
                "size": 10,
                "contexts": {
                    "product_type": "casual",
                    "product_brand": "reebok"
                }
            }
        }
    }
}

Executing the above query will show error message as below

"Unknown context name[product_brand], must be one of 3"

This error message is not very informative as it only convey the number of context supported. However printing all the accepted contexts will be more informative, like below:

"Unknown context name[product_brand], must be one of [product_size, product_color, product_type]"

This pull request fixes the issue mentioned.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[nik9000]

I left a minor thing but it makes sense to me. If you'll make the change I can see about merging.

[nik9000]

I think we'd better sort the list of keys before adding it to the string so we get consistent error messages. This'll make testing much easier and be marginally easier on the eyes.

[nik9000]

Can you add a space between "name" and "["?

[nilabhsagar]

Thanks @nik9000 I agree, sorting will have consistent behaviour. I will do both the changes as suggested and commit.

[nilabhsagar]

@nik9000 changes done. Please verify

[nik9000]

Looks good to me. I'll work on getting this merged.

[nik9000]

elasticmachine, test this please.

[nik9000]

Thanks @nilabhsagar! I've merged to master and an backporting to 5.x.

[nilabhsagar]

Thanks @nik9000!