Allow methods to be used for flexible searchable_fields

[tiller1010]

Summary

Allow methods to be used for flexible $searchable_fields. A method name can be added as a $searchable_fields key. The method should return an array of fields that can be used when using this search field in a GridFieldFilterHeader.

Testing

• create a DataObject with $searchable_fields
• set a key to a method name that returns the appropriate search fields in an array (see comments in SearchContext)
• create records that use the searchable fields returned in your method
• confirm the search results will show any records that match at least one of the returned search fields

Concerns

• It was necessary for me to define the field type in the searchable field options.

[emteknetnz]

Could you help me understand what this is supposed to do from a UI perspective? Based on

                        public function getSearchableFirstName($queryName = '')
                        {
                            return [
                                'Customer.FirstName' => $queryName,
                                'ShippingAddress.FirstName' => $queryName,
                            ];
                        }

I'm assuming that when a user searches on FirstName it would now search on both the Customer.FirstName database field as well as the ShippingAddress.FirstName field?

[tiller1010]

@emteknetnz

I'm assuming that when a user searches on FirstName it would now search on both the Customer.FirstName database field as well as the ShippingAddress.FirstName field?

That's exactly right.

[emteknetnz]

I'm finding the syntax of this confusing. I don't think there's a need for a separate method as we're limited to what's in the database, and we cannot use dynamic content in this context i.e. filtering a DataList (as far as I'm aware, maybe I'm wrong)

I think putting everything in $searchable_fields would make more sense

private static $searchable_fields = [
    'CustomFirstName' => [
        'title' => 'First Name',
        'field' => TextField::class,
        'filter' => 'PartialMatchFilter',
        'match_any' => [
            'Customer.FirstName,
            'ShippingAddress.FirstName'
        ]
    ]
]

To merge this PR we'll also need unit test coverage added to this PR, as well as documentation about this additional functionality updated here rather than as an code comment as it is now

[michalkleiner]

Looks ok from my perspective, thanks!

[emteknetnz]

Great work.

I've tested this locally to confirm it works as expected

Some very minor changes then I'm happy to merge

There's some PHPCS linting to fix as well https://app.travis-ci.com/github/silverstripe/silverstripe-framework/jobs/558865008

[emteknetnz]

I don't think we need these inline comments explaining the code or the line breaks above them