Make copy of cached fields so they don't get polluted

src/Foundatio.Parsers.SqlQueries/SqlQueryParser.cs

@@ -86,7 +86,7 @@ public SqlQueryVisitorContext GetContext(IEntityType entityType)
         Configuration.SetValidationOptions(validationOptions);
         return new SqlQueryVisitorContext
         {
-            Fields = fields,
+            Fields = fields.ToList(),
             ValidationOptions = validationOptions
         };
     }