Reimplement compound index

Author: imnasnainaec

Backend/Repositories/SemanticDomainCountRepository.cs

@@ -10,20 +10,33 @@ namespace BackendFramework.Repositories
 {
     /// <summary> Atomic database functions for <see cref="ProjectSemanticDomainCount"/>s. </summary>
     [ExcludeFromCodeCoverage]
-    public class SemanticDomainCountRepository(IMongoDbContext dbContext) : ISemanticDomainCountRepository
+    public class SemanticDomainCountRepository : ISemanticDomainCountRepository
     {
-        private readonly IMongoCollection<ProjectSemanticDomainCount> _counts =
-            dbContext.Db.GetCollection<ProjectSemanticDomainCount>("SemanticDomainCountCollection");
+        private readonly IMongoCollection<ProjectSemanticDomainCount> _counts;
 
         private const string otelTagName = "otel.SemanticDomainCountRepository";
 
+        public SemanticDomainCountRepository(IMongoDbContext dbContext)
+        {
+            _counts = dbContext.Db.GetCollection<ProjectSemanticDomainCount>("SemanticDomainCountCollection");
+
+            // Create unique compound index on (ProjectId, DomainId) to optimize queries and enforce uniqueness
+            var indexKeys =
+                Builders<ProjectSemanticDomainCount>.IndexKeys.Ascending(p => p.ProjectId).Ascending(p => p.DomainId);
+            var indexModel = new CreateIndexModel<ProjectSemanticDomainCount>(indexKeys, new() { Unique = true });
+
+            // Create the index if it doesn't already exist
+            _counts.Indexes.CreateOne(indexModel);
+        }
+
         private static FilterDefinition<ProjectSemanticDomainCount> ProjectFilter(string projectId)
         {
             var filterDef = new FilterDefinitionBuilder<ProjectSemanticDomainCount>();
             return filterDef.Eq(c => c.ProjectId, projectId);
         }
 
-        private static FilterDefinition<ProjectSemanticDomainCount> ProjectDomainFilter(string projectId, string domainId)
+        private static FilterDefinition<ProjectSemanticDomainCount> ProjectDomainFilter(
+            string projectId, string domainId)
         {
             var filterDef = new FilterDefinitionBuilder<ProjectSemanticDomainCount>();
             return filterDef.And(filterDef.Eq(c => c.ProjectId, projectId), filterDef.Eq(c => c.DomainId, domainId));