Mapping stores

Compiling time type mapping

When instances of a compile time data type (.NET classes excluding EPiServer.Data.Dynamic.PropertyBag and classes implementing System.IEnumerable) are saved in the Dynamic Data Store, their “inline” properties are mapped to columns in the “big table”. This is known logically as a store.

Default mapping

The default algorithm for mapping .NET classes (excluding EPiServer.Data.Dynamic.PropertyBag and classes implementing System.IEnumerable) to a store is as follows:

• Property must have a getter and setter
• Property must be marked public (although the setter can marked non-public if desired)
• All other properties are ignored and not saved in the Dynamic Data Store

Custom mapping

You can override the default mapping behavior. This is useful if you do not want certain public properties to be mapped or do want certain non-public properties to be mapped.

To use custom mapping, you need to add the System.Runtime.Serialization.DataContactAttribute to your class definition. In this case, only properties marked with the System.Runtime.Serialization.DataMemberAttribute will be mapped and saved in the Dynamic Data Store regardless of the accessibility status. They must still however have both a getter and setter.

See the MappingWithDataContract class in the DDS sample project for examples.

You may want to save an object of an existing class that has already been marked with DataContactAttribute and its member properties with DataMemberAttribute. One problem might be that the use of these properties does not match the desired behavior when an object instance is saved in the Dynamic Data Store. In these cases, you can also add the EPiServerDataContractAttribute to the class definition and EPiServerDataMemberAttribute to the properties to be saved. The Dynamic Data Store will use these attributes in preference to the Microsoft ones to resolve the conflict.

See the MappingWithEPiServerDataContract class in the DDS sample project for examples.

Type handlers

Some classes in the .NET Framework do not have properties that the Dynamic Data Store can use to extract the value, save to the database and then re-inflate an instance with the value from the database. If you want to use such a type as a property on a class that will be saved to the Dynamic Data Store, you need to register a Type Handler for it.

A good example of this is the System.Uri class. This class only has read-only properties and therefore saving an instance to the Dynamic Data Store without a Type Handler is meaningless as no data will be stored for it.

See the MappingWithTypeHandler class in the DDS sample project for examples.

Mapping runtime data type (PropertyBag)

Properties saved using PropertyBags are mapped as if the properties were public members properties on a normal .NET class. PropertyBags can be mapped in the following ways:

• Implicit is the first time a PropertyBag is saved to a new store the store mappings are inferred from the properties in the PropertyBag.
• Explicit is a mapping dictionary which is passed to the DynamicDataStoreFactory.CreateStore method detailing the store mapping. The main advantage of this is that all properties that can be potentially saved in this store are mapped, as opposed to the first time a PropertyBag is saved which may not have all potential values.

See the ImplicitDynamicMapping and ExplicitDynamicMapping classes in the DDS sample project for examples. 

Storing remapping

From time to time you may need to change the structure of your data. This can mean adding, removing or changing properties on your .NET classes or PropertyBags.

The Dynamic Data Store is quite flexible when it comes to accepting changes to types that have been saved in a store.

You can remap .NET classes to stores in the following ways:

• Using attributes on the class.
• Using attributes via the StoreDefinition class. Remapping stores that are not represented by a .NET class can only be done with the StoreDefinition class.

Remapping using class attributes

A .NET class whose instances will be saved in the Dynamic Data Store can be optionally decorated with the EPiServerDataStoreAttribute. In these cases, set the AutomaticallyRemapStore property to true. When the Episerver application starts, it scans for all classes with this attribute and automatically remaps the .NET class to the store, if needed. Any properties that have been renamed MUST be marked with the EPiServerDataPropertyRenameAttribute attribute, otherwise the remap treats them as if one property was removed (with the old name) and one added (with the new name).

Remapping using StoreDefinition class

To remap a store, obtain the store definition of a store either via the StoreDefinition property of a DynamicDataStore instance or via the StoreDefinition.Get method. You can then call the Rename and Remap methods to update the store's mappings. Note that you should call Rename before Remap, otherwise properties that have been renamed in the data type are treated as if one property has been removed and one added. Finally, call the CommitChanges method of StoreDefinition to update the store's meta information in the database. If a DynamicDataStore instance reference is held, then its Refresh method should be called to align its in-memory copy of the store definition with the one committed to disk.

Remapping rules

These rules are followed when remapping stores:

1. Properties removed from the type definition are removed from the store. Note: The data itself for the removed property remain intact in the big table. It is only the “view” of the data that is changed.
2. Properties added to the type definition are added to the store.
3. Properties with the same name but different data types are checked for compatibility. In the case where both old and new property data types are ‘inline’, the database used must be able to convert from the old database data type to the new database data type. If a collection or reference type is changed, then the new property type must either be assignable (using System.Type.IsAssignableFrom) or the old type must be convertible to the new type. The old type supports System.IConvertible and the System.Convert.ChangeType succeeds in converting an instance of the old type to an instance of the new type.

See the StoreReMapping class for examples of store re-mapping and the PropertyReName class for how to update mappings when a property has been renamed on a type, both in the Dynamic Data Store SDK.

Mapping types to specific stores

It can be convenient to save instances of a Type in the same store, regardless of where those instances are in an object graph. You have the following options:

1. Global Mapping. Use the EPiServer.Data.Dynamic.GlobalTypeToStoreMap.Instance methods to add and remove a mapping or add an EPiServer.Data.Dynamic.EPiServerDataStoreAttribute to the .NET class. When this is used, all instances of the registered Type are saved in the store with the specified name.
2. Local Mapping. A delegate is passed to the DynamicDataStore or DynamicDataStore<T> Save method. This delegate will be called for all “reference” properties and collection items that are references. The delegate should return the name of the store to save the item in. This allows extra flexibility to steer instances of a Type into different stores depending on the context and how they are used.

Example:

1. A global Type to Store mapping is added so all instances of the Type “Person” are saved in a store called “People”.
2. A store is created/obtained with the name “MyPeople”.
3. An instance of a Person Type is saved in the store obtained in step 2. This instance has properties that are also Person instances.

Result: The top level Person is saved in the “MyPeople” store but all other instances of the Person Type in the object graph are saved in the “People” store (because of the global mapping).

In order to adhere to the global Type to Store mappings you should create or obtain your top level store by calling DynamicDataStoreFactory.Create or GetStore with just the type and not a store name.

See the UsingGlobalTypeToStoreMapping and UsingLocalTypeToStoreMapping classes in the DDS sample project for more details.

Mapping stores to custom big tables

In the same way as it may be convenient to map a Type to a Store, it may also prove convenient to map a store to a custom big table. See the Big Table section in the Dynamic Data Store topic for more details.

The UsingGlobalStoreToTableMapping class in the DDS sample project demonstrates this technique.

Last updated: Jan 05, 2016