Helper methods to expose field mappings (#1918)

## Why make this change?
For customers consuming dab, it maybe useful to store field mappings
internally and surface them to their end users. Currently these are
private dictionaries and hence need to be regenerated if wanting to show
on consumer side.

## What is this change?
Exposing the dictionaries publicly through readonlydictionary.

## How was this tested?
Existing unit tests should cover all scenarios.

---------

Co-authored-by: Abhishek  Kumar <[email protected]>

Author: rohkhann

src/Core/Services/MetadataProviders/CosmosSqlMetadataProvider.cs

@@ -276,7 +276,7 @@ public bool TryGetBackingColumn(string entityName, string field, [NotNullWhen(tr
             return true;
         }
 
-        public IDictionary<string, DatabaseObject> GetEntityNamesAndDbObjects()
+        public IReadOnlyDictionary<string, DatabaseObject> GetEntityNamesAndDbObjects()
         {
             throw new NotImplementedException();
         }

src/Core/Services/MetadataProviders/ISqlMetadataProvider.cs

@@ -121,7 +121,7 @@ bool VerifyForeignKeyExistsInDB(
         /// Returns a dictionary of (EntityName, DatabaseObject).
         /// </summary>
         /// <returns></returns>
-        public IDictionary<string, DatabaseObject> GetEntityNamesAndDbObjects();
+        public IReadOnlyDictionary<string, DatabaseObject> GetEntityNamesAndDbObjects();
 
         /// <summary>
         /// Gets Partition Key Path of a database container.

src/Core/Services/MetadataProviders/SqlMetadataProvider.cs

@@ -222,7 +222,7 @@ public virtual bool TryGetEntityNameFromPath(string entityPathName, [NotNullWhen
         }
 
         /// <inheritdoc />
-        public IDictionary<string, DatabaseObject> GetEntityNamesAndDbObjects()
+        public IReadOnlyDictionary<string, DatabaseObject> GetEntityNamesAndDbObjects()
         {
             return EntityToDatabaseObject;
         }
@@ -284,6 +284,38 @@ public async Task InitializeAsync()
             _logger.LogTrace($"Done inferring Sql database schema in {timer.ElapsedMilliseconds}ms.");
         }
 
+        /// <summary>
+        /// Given entity name, gets the entity to column mappings if present.
+        /// </summary>
+        public bool TryGetEntityToColumnMappings(string entityName, [NotNullWhen(true)] out IReadOnlyDictionary<string, string>? mappings)
+        {
+            Dictionary<string, string>? entityToColumnMappings;
+            mappings = null;
+            if (EntityExposedNamesToBackingColumnNames.TryGetValue(entityName, out entityToColumnMappings))
+            {
+                mappings = entityToColumnMappings;
+                return true;
+            }
+
+            return false;
+        }
+
+        /// <summary>
+        /// Given entity name, gets the column to entity mappings if present.
+        /// </summary>
+        public bool TryGetColumnToEntityMappings(string entityName, [NotNullWhen(true)] out IReadOnlyDictionary<string, string>? mappings)
+        {
+            Dictionary<string, string>? columntoEntityMappings;
+            mappings = null;
+            if (EntityBackingColumnsToExposedNames.TryGetValue(entityName, out columntoEntityMappings))
+            {
+                mappings = columntoEntityMappings;
+                return true;
+            }
+
+            return false;
+        }
+
         /// <summary>
         /// Log Primary key information. Function only called when not
         /// in a hosted scenario. Log relevant information about Primary keys

src/Service.Tests/Unittests/SqlMetadataProviderUnitTests.cs

@@ -234,6 +234,30 @@ public async Task CheckCorrectParsingForStoredProcedure()
             TestHelper.UnsetAllDABEnvironmentVariables();
         }
 
+        /// <summary>
+        /// <code>Do: </code> Load runtimeConfig and set up the source fields for the entities.
+        /// <code>Check: </code>  Verifies that source object is correctly parsed.
+        /// </summary>
+        [TestMethod, TestCategory(TestCategory.MSSQL)]
+        public async Task CheckGetFieldMappings()
+        {
+            DatabaseEngine = TestCategory.MSSQL;
+            TestHelper.SetupDatabaseEnvironment(DatabaseEngine);
+            RuntimeConfig runtimeConfig = SqlTestHelper.SetupRuntimeConfig();
+            RuntimeConfigProvider runtimeConfigProvider = TestHelper.GenerateInMemoryRuntimeConfigProvider(runtimeConfig);
+            SetUpSQLMetadataProvider(runtimeConfigProvider);
+
+            await _sqlMetadataProvider.InitializeAsync();
+
+            MsSqlMetadataProvider metadataProvider = (MsSqlMetadataProvider)_sqlMetadataProvider;
+            Assert.IsFalse(metadataProvider.TryGetColumnToEntityMappings("InvalidEntity", out _), "Column to entity mappings should not exist for invalid entity.");
+            Assert.IsFalse(metadataProvider.TryGetEntityToColumnMappings("invalidEntity", out _), "Entity to column mappings should not exist for invalid entity.");
+            Assert.IsTrue(metadataProvider.TryGetEntityToColumnMappings("Publisher", out IReadOnlyDictionary<string, string> _), "Entity to column mappings should exist for valid entity.");
+            Assert.IsTrue(metadataProvider.TryGetColumnToEntityMappings("Publisher", out IReadOnlyDictionary<string, string> _), "Column to entity mappings should exist for valid entity.");
+
+            TestHelper.UnsetAllDABEnvironmentVariables();
+        }
+
         [DataTestMethod, TestCategory(TestCategory.MSSQL)]
         [DataRow("/mygql", "/graphql", true, DisplayName = "Entity Rest path conflicts with default path /graphql")]
         [DataRow("/mygql", "/mygql", true, DisplayName = "Entity Rest path conflicts with configured GraphQL path")]