Add relationship support for entities of object type view

src/Core/Configurations/RuntimeConfigValidator.cs

@@ -323,10 +323,22 @@ public void ValidateRelationshipConfigCorrectness(RuntimeConfig runtimeConfig)
             if (entity.Source.Type is not EntitySourceType.Table && entity.Relationships is not null
                 && entity.Relationships.Count > 0)
             {
-                HandleOrRecordException(new DataApiBuilderException(
-                        message: $"Cannot define relationship for entity: {entityName}",
-                        statusCode: HttpStatusCode.ServiceUnavailable,
-                        subStatusCode: DataApiBuilderException.SubStatusCodes.ConfigValidationError));
+                // Views are allowed to have relationships only for MSSQL and DWSQL databases.
+                // Stored procedures cannot have relationships.
+                string databaseNameForEntity = runtimeConfig.GetDataSourceNameFromEntityName(entityName);
+                DatabaseType dbType = runtimeConfig.GetDataSourceFromDataSourceName(databaseNameForEntity).DatabaseType;
+
+                bool isViewWithRelationshipAllowed = entity.Source.Type is EntitySourceType.View
+                    && (dbType is DatabaseType.MSSQL or DatabaseType.DWSQL);
+
+                if (!isViewWithRelationshipAllowed)
+                {
+                    HandleOrRecordException(new DataApiBuilderException(
+                            message: $"Cannot define relationship for entity: {entityName}. " +
+                                     $"Relationships are only supported for tables, or for views when using MSSQL or DWSQL databases.",
+                            statusCode: HttpStatusCode.ServiceUnavailable,
+                            subStatusCode: DataApiBuilderException.SubStatusCodes.ConfigValidationError));
+                }
             }
 
             string databaseName = runtimeConfig.GetDataSourceNameFromEntityName(entityName);
@@ -1067,10 +1079,20 @@ public void ValidateRelationships(RuntimeConfig runtimeConfig, IMetadataProvider
                     continue;
                 }
 
-                DatabaseTable sourceDatabaseObject = (DatabaseTable)sourceObject;
-                DatabaseTable targetDatabaseObject = (DatabaseTable)targetObject;
+                // Source and target objects can be tables or views (for MSSQL/DWSQL)
+                DatabaseObject sourceDatabaseObject = sourceObject;
+                DatabaseObject targetDatabaseObject = targetObject;
+
+                // For RelationShipPair comparisons, we convert to DatabaseTable since the comparison
+                // only uses schema and name (not the actual type)
+                DatabaseTable sourceDatabaseTable = sourceDatabaseObject as DatabaseTable
+                    ?? new DatabaseTable(sourceDatabaseObject.SchemaName, sourceDatabaseObject.Name);
+                DatabaseTable targetDatabaseTable = targetDatabaseObject as DatabaseTable
+                    ?? new DatabaseTable(targetDatabaseObject.SchemaName, targetDatabaseObject.Name);
+
                 if (relationship.LinkingObject is not null)
                 {
+                    // Linking object must remain a table
                     (string linkingTableSchema, string linkingTableName) = sqlMetadataProvider.ParseSchemaAndDbTableName(relationship.LinkingObject)!;
                     DatabaseTable linkingDatabaseObject = new(linkingTableSchema, linkingTableName);
 
@@ -1103,8 +1125,8 @@ public void ValidateRelationships(RuntimeConfig runtimeConfig, IMetadataProvider
                         string sourceDBOName = sqlMetadataProvider.EntityToDatabaseObject[entityName].FullName;
                         string targetDBOName = sqlMetadataProvider.EntityToDatabaseObject[relationship.TargetEntity].FullName;
                         string cardinality = relationship.Cardinality.ToString().ToLower();
-                        RelationShipPair linkedSourceRelationshipPair = new(linkingDatabaseObject, sourceDatabaseObject);
-                        RelationShipPair linkedTargetRelationshipPair = new(linkingDatabaseObject, targetDatabaseObject);
+                        RelationShipPair linkedSourceRelationshipPair = new(linkingDatabaseObject, sourceDatabaseTable);
+                        RelationShipPair linkedTargetRelationshipPair = new(linkingDatabaseObject, targetDatabaseTable);
                         ForeignKeyDefinition? fKDef;
                         string referencedSourceColumns = relationship.SourceFields is not null ? string.Join(",", relationship.SourceFields) :
                             sqlMetadataProvider.PairToFkDefinition!.TryGetValue(linkedSourceRelationshipPair, out fKDef) ?
@@ -1150,8 +1172,8 @@ public void ValidateRelationships(RuntimeConfig runtimeConfig, IMetadataProvider
 
                 if (relationship.LinkingObject is null && !_runtimeConfigProvider.IsLateConfigured)
                 {
-                    RelationShipPair sourceTargetRelationshipPair = new(sourceDatabaseObject, targetDatabaseObject);
-                    RelationShipPair targetSourceRelationshipPair = new(targetDatabaseObject, sourceDatabaseObject);
+                    RelationShipPair sourceTargetRelationshipPair = new(sourceDatabaseTable, targetDatabaseTable);
+                    RelationShipPair targetSourceRelationshipPair = new(targetDatabaseTable, sourceDatabaseTable);
                     string sourceDBOName = sqlMetadataProvider.EntityToDatabaseObject[entityName].FullName;
                     string targetDBOName = sqlMetadataProvider.EntityToDatabaseObject[relationship.TargetEntity].FullName;
                     string cardinality = relationship.Cardinality.ToString().ToLower();

src/Core/Services/MetadataProviders/CosmosSqlMetadataProvider.cs

@@ -498,8 +498,8 @@ public IQueryBuilder GetQueryBuilder()
         }
 
         public bool VerifyForeignKeyExistsInDB(
-            DatabaseTable databaseTableA,
-            DatabaseTable databaseTableB)
+            DatabaseObject databaseObjectA,
+            DatabaseObject databaseObjectB)
         {
             throw new NotImplementedException();
         }

src/Core/Services/MetadataProviders/ISqlMetadataProvider.cs

@@ -28,8 +28,8 @@ public interface ISqlMetadataProvider
         string GetSchemaName(string entityName);
 
         bool VerifyForeignKeyExistsInDB(
-            DatabaseTable databaseObjectA,
-            DatabaseTable databaseObjectB);
+            DatabaseObject databaseObjectA,
+            DatabaseObject databaseObjectB);
 
         /// <summary>
         /// Obtains the underlying source object's name (SQL table or Cosmos container).