The method 'Concat' is not supported. Declaring type: System.Linq.Queryable

I recently implemented this type of requirement in Commerce and same can be used here also.

Use the first predicate to get UK results

query1 = query.Where(i => i.region_names.Contains("UK"));
var results = query1.GetResults();

Use the second predicate to get non UK results

query2 = query.Where(i => !i.region_names.Contains("UK"));
var results = query2.GetResults();

Finally perform Union of the results

var results3 = results1.Union(results2);
SearchResults<SearchResultItem> fianlResults = new SearchResults<SearchResultItem>(results3, results3.Count());

This should give you the expected output. Please let me know if you face any issues

You can refer to my blog here for more details : https://codeandlearnspot.wordpress.com/2020/04/05/sitecore-solr-search-improve-relevancy-boosting-keyword-splitting/

As specified by the documentation found here https://doc.sitecore.com/developers/90/sitecore-experience-manager/en/linq-to-sitecore.html, the concat method is not supported.

You can rather do a single query where the region_names is not empty and select the items you want from the result.

query = query.Where(i => !string.IsNullOrEmpty(i.region_names));
var results = query.GetResults();

var finalResult = results.Hits.Where(i => i.Document.region_names.Contains("UK")).ToList();
finalResult.AddRange(results.Hits.Where(i => !i.Document.region_names.Contains("UK")).ToList());

Update: For pagination feature you can use skip and take operations on the finalResult list.

var newList = finalResult.Skip(x).Take(y);

• x will be the number of contents you want to skip
• y will be the number of contents to display per page

LINQ to Sitecore provides access to search the indexes, using standard LINQ queries in the same way that other LINQ providers, such as LINQ to SQL and LINQ to Objects, work. It uses the standard IQueryable interface and supports most of the available operations. But the LINQ layer does not implement all IQueryable methods. below methods are not supported:

• Average
• Concat
• Facets
• FirstOrDefault
• GroupBy
• Intersect
• LastOrDefault
• Match
• OrderByDescending
• Reverse
• SingleOrDefault
• SkipWhile
• Sum
• TakeWhile

If these methods are called, a NotSupportedException or InvalidOperationException exception is thrown at runtime. Based on your requirement you may need to fire two queries or need to perform logic after getting the results.