Add helper function that can prevent errors and reduce log noise (#142)

* Add helper function that can prevent errors and reduce log noise

* Fix reference to a file in readme

gateway/README.md

@@ -29,7 +29,7 @@ message GetMessageRequest {
   string user_id = 2;
 }
 ```
-This enables the following two alternative HTTP JSON to RPC mappings: 
+This enables the following two alternative HTTP JSON to RPC mappings:
 
 | HTTP Verb | REST Endpoint                     | RPC                                                 |
 | ----------|-----------------------------------|---------------------------------------------------- |
@@ -287,6 +287,23 @@ Response with results and service-defined results tag `rpz_hits`
 }
 ```
 
+## Query String Filtering
+When using the collection operators with the grpc-gateway, extraneous errors may
+be logged during rpcs as the query string is parsed that look like this:
+```
+field not found in *foo.ListFoobarRequest: _order_by
+```
+and the usage of any of the collection operator field names without the leading
+underscore (`order_by`, `filter`,... instead of `_order_by`, `filter`,...) in
+query strings may result in the error `unsupported field type reflect.Value`,
+being returned.
+
+This can be resolved by overwriting the default filter for each rpc with these
+operators using the one defined in [filter.go](filter.go).
+```golang
+filter_Foobar_List_0 = gateway.DefaultQueryFilter
+```
+
 ## Errors
 
 ### Format

gateway/filter.go

@@ -0,0 +1,27 @@
+package gateway
+
+import "github.com/grpc-ecosystem/grpc-gateway/utilities"
+
+// DefaultQueryFilter can be set to override the filter_{service}_{rpc}_{num}
+// field in generated .pb.gw.go files to prevent parse errors in the gateway
+// and potentially reduce log noise due to unrecognized fields
+var DefaultQueryFilter = utilities.NewDoubleArray(defaultFilterFields)
+
+var defaultFilterFields = [][]string{
+	// collection ops and the expected names used for the collection ops objects in requests
+	{"paging"}, {limitQueryKey}, {offsetQueryKey}, {pageTokenQueryKey},
+	{"order_by"}, {sortQueryKey},
+	{"fields"}, {fieldsQueryKey},
+	{"filter"}, {filterQueryKey},
+}
+
+// QueryFilterWith will add extra fields to the standard fields in the default
+// filter.
+func QueryFilterWith(extraFields []string) *utilities.DoubleArray {
+	qf := make([][]string, len(defaultFilterFields)+len(extraFields))
+	copy(qf, defaultFilterFields)
+	for _, f := range extraFields {
+		qf = append(qf, []string{f})
+	}
+	return utilities.NewDoubleArray(qf)
+}

gateway/response.go

@@ -4,11 +4,12 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
+	"net/http"
+
 	"github.com/golang/protobuf/proto"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/grpclog"
-	"io"
-	"net/http"
 
 	"github.com/grpc-ecosystem/grpc-gateway/runtime"
 )