public bool Delete(Guid relationId)
        {
            DbEntityRelation relation = Read(relationId);

            if (relation == null)
            {
                throw new StorageException("There is no record with specified relation id.");
            }

            List <DbEntity> entities = DbContext.Current.EntityRepository.Read();

            DbEntity originEntity = entities.FirstOrDefault(e => e.Id == relation.OriginEntityId);
            DbEntity targetEntity = entities.FirstOrDefault(e => e.Id == relation.TargetEntityId);

            DbBaseField originField = originEntity.Fields.FirstOrDefault(f => f.Id == relation.OriginFieldId);
            DbBaseField targetField = targetEntity.Fields.FirstOrDefault(f => f.Id == relation.TargetFieldId);

            string originTableName = $"rec_{originEntity.Name}";
            string targetTableName = $"rec_{targetEntity.Name}";

            try
            {
                using (DbConnection con = DbContext.Current.CreateConnection())
                {
                    con.BeginTransaction();

                    NpgsqlCommand command = con.CreateCommand("DELETE FROM entity_relations WHERE id=@id;");

                    var parameterId = command.CreateParameter() as NpgsqlParameter;
                    parameterId.ParameterName = "id";
                    parameterId.Value         = relationId;
                    parameterId.NpgsqlDbType  = NpgsqlDbType.Uuid;
                    command.Parameters.Add(parameterId);

                    try
                    {
                        command.ExecuteNonQuery();

                        if (originField.Name != "id")
                        {
                            DbRepository.DropIndex($"idx_r_{relation.Name}_{originField.Name}");
                        }
                        if (targetField.Name != "id")
                        {
                            DbRepository.DropIndex($"idx_r_{relation.Name}_{targetField.Name}");
                        }

                        if (relation.RelationType == EntityRelationType.ManyToMany)
                        {
                            DbRepository.DeleteNtoNRelation(relation.Name, originTableName, targetTableName);
                        }
                        else
                        {
                            DbRepository.DeleteRelation(relation.Name, targetTableName);
                        }

                        con.CommitTransaction();
                        return(true);
                    }
                    catch (Exception)
                    {
                        con.RollbackTransaction();
                    }
                }

                return(false);
            }
            finally
            {
                Cache.Clear();
            }
        }
        public bool Create(DbEntityRelation relation)
        {
            try
            {
                if (relation == null)
                {
                    throw new ArgumentNullException("relation");
                }

                List <DbParameter> parameters = new List <DbParameter>();

                JsonSerializerSettings settings = new JsonSerializerSettings {
                    TypeNameHandling = TypeNameHandling.Auto
                };

                DbParameter parameterId = new DbParameter();
                parameterId.Name  = "id";
                parameterId.Value = relation.Id;
                parameterId.Type  = NpgsqlDbType.Uuid;
                parameters.Add(parameterId);

                DbParameter parameterJson = new DbParameter();
                parameterJson.Name  = "json";
                parameterJson.Value = JsonConvert.SerializeObject(relation, settings);
                parameterJson.Type  = NpgsqlDbType.Json;
                parameters.Add(parameterJson);

                List <DbEntity> entities = DbContext.Current.EntityRepository.Read();

                DbEntity originEntity = entities.FirstOrDefault(e => e.Id == relation.OriginEntityId);
                DbEntity targetEntity = entities.FirstOrDefault(e => e.Id == relation.TargetEntityId);

                DbBaseField originField = originEntity.Fields.FirstOrDefault(f => f.Id == relation.OriginFieldId);
                DbBaseField targetField = targetEntity.Fields.FirstOrDefault(f => f.Id == relation.TargetFieldId);

                string originTableName = $"rec_{originEntity.Name}";
                string targetTableName = $"rec_{targetEntity.Name}";

                using (DbConnection con = DbContext.Current.CreateConnection())
                {
                    con.BeginTransaction();

                    try
                    {
                        bool result = DbRepository.InsertRecord("entity_relations", parameters);

                        if (relation.RelationType == EntityRelationType.ManyToMany)
                        {
                            DbRepository.DeleteNtoNRelation(relation.Name, originTableName, targetTableName);
                            DbRepository.CreateNtoNRelation(relation.Name, originTableName, originField.Name, targetTableName, targetField.Name);
                        }
                        else
                        {
                            DbRepository.DeleteRelation(relation.Name, targetTableName);
                            if (originField.Name != "id")
                            {
                                DbRepository.DropIndex($"idx_r_{relation.Name}_{originField.Name}");
                            }
                            if (targetField.Name != "id")
                            {
                                DbRepository.DropIndex($"idx_r_{relation.Name}_{targetField.Name}");
                            }

                            DbRepository.CreateRelation(relation.Name, originTableName, originField.Name, targetTableName, targetField.Name);
                            if (originField.Name != "id")
                            {
                                DbRepository.CreateIndex($"idx_r_{relation.Name}_{originField.Name}", originTableName, originField.Name);
                            }
                            if (targetField.Name != "id")
                            {
                                DbRepository.CreateIndex($"idx_r_{relation.Name}_{targetField.Name}", targetTableName, targetField.Name);
                            }
                        }

                        con.CommitTransaction();
                        return(true);
                    }
                    catch
                    {
                        con.RollbackTransaction();
                        throw;
                    }
                }
            }
            finally
            {
                Cache.Clear();
            }
        }