I tested out the contains operator on string fields today in GraphQL. It is actually case sensitive, and this is counter-intuitive since I have connected the GraphQL database to a SQL database, which performs usually a case insensitive search with the 'contains' operator (using 'LIKE' operator
under the hood).
The following adjustments need to be made to make it work :
First off, define a class inheriting
QueryableStringOperationHandler
using HotChocolate.Data.Filters;
using HotChocolate.Data.Filters.Expressions;
using HotChocolate.Language;
using System.Linq.Expressions;
using System.Reflection;
namespace AspNetGraphQLDemoV2.Server
{
public class QueryableStringInvariantContainsHandler : QueryableStringOperationHandler
{
private static readonly MethodInfo _contains = typeof(string).GetMethod(nameof(string.Contains), new[] { typeof(string) })!;
public QueryableStringInvariantContainsHandler(InputParser inputParser) : base(inputParser)
{
}
protected override int Operation => DefaultFilterOperations.Contains;
public override Expression HandleOperation(QueryableFilterContext context, IFilterOperationField field,
IValueNode value, object? parsedValue)
{
Expression property = context.GetInstance();
if (parsedValue is string str)
{
var toLower = Expression.Call(property, typeof(string).GetMethod("ToLower", Type.EmptyTypes)!); //get the ToLower method of string class via reflection. The Type.EmptyTypes will retrieve the method overload of ToLower which accept no arguments.
var finalExpression = Expression.Call(toLower, _contains, Expression.Constant(str.ToLower()));
return finalExpression;
}
throw new InvalidOperationException();
}
}
}
We overload the Operation to 'Contains' so we are going to adjust how we treat the expression tree of GraphQL, overriding the
HandleOperation. This is similar to the QueryableStringOperationHandler presented here:
https://chillicream.com/docs/hotchocolate/api-reference/extending-filtering, in our case we support the contains method instead.
The finalExpression 'DebugView' evaluates to :
.Call (.Call ($_s0.OfficialName).ToLower()).Contains("tind")
To actually use this adaption of filtering of string properties in HotChocolate, we do the following in program.cs (startup class in .net 6) to not only add filtering support to HotChocolate, but also add a filter convention extension first to make it easier to register and avoid adding cluttering to the startup code in program.cs :
using HotChocolate.Data.Filters;
using HotChocolate.Data.Filters.Expressions;
namespace AspNetGraphQLDemoV2.Server
{
public class FilterConventionExtensionForInvariantContainsStrings : FilterConventionExtension
{
protected override void Configure(IFilterConventionDescriptor descriptor)
{
descriptor.AddProviderExtension(new QueryableFilterProviderExtension(
y => y.AddFieldHandler<QueryableStringInvariantContainsHandler>()));
}
}
}
You can see an example how I register this case insensitive contains filter in the program.cs example code below. Note both the usage of
.AddFiltering() and the
.AddConvention() call.
var builder = WebApplication.CreateBuilder(args);
var connectionString = builder.Configuration.GetConnectionString("MountainsV2Db");
builder.Services
.AddDbContext<MountainDbContext>(options =>
{
options.UseSqlServer(connectionString);
})
.AddCors()
.AddGraphQLServer()
.AddProjections()
.AddFiltering()
.AddConvention<IFilterConvention, FilterConventionExtensionForInvariantContainsStrings>()
.AddSorting()
.RegisterDbContext<MountainDbContext>()
.AddQueryType<MountainQueries>()
.AddMutationType<MountainMutations>()
.AddSubscriptionType<MountainSubscriptions>()
.AddInMemorySubscriptions();
var app = builder.Build();
Now, sample .graphql file (containing a GraphQL query) that shows how the new filtering capability can be used :
query {
mountains (where: { officialName: {contains: "TiND"}}) {
officialName
}
}
The backend code retrieves a list of data from a table and I have added the [UseFiltering] attribute to specify for HotChocolate that filtering should be supported to the method.
public class MountainQueries
{
[UseFiltering]
[UseSorting]
public async Task<List<Mountain>> GetMountains([Service] MountainDbContext mountainDb)
{
return await mountainDb.Mountains.ToListAsync();
}
//..
