The DataMapper Pattern is a well documented and defined pattern for abstracting away an object’s storage from its in-memory representation. For the most part the pattern explicitly defines the domain of the pattern to be relational databases and object’s that map to there schema, however the pattern is adaptable to non-relational data-stores as well. In fact to applicable to any situation where the need to map records (related data units) to in memory objects; regardless of structure or underlying store.
Imagine, for instance, a service that is a façade for a complex matrix of unstructured key value pairs and a protocol for clients to query the service for tuples. For example:
For the make ‘honda’, give me all ‘years’
For the article ‘2311’, give me ‘title’, ‘description’, ‘author’, ‘publication_date’
This is not too much different from any relational database model, with the exception that there is no direct joining occurring to return back columns that occur in different tables and relate them to the input set. It’s basically just a giant hash-map of name/values pairs with an undefined structure…i.e. there is no way to directly map it to a domain model; what you supply as inputs, defines the structure of your output.
That’s where the DataMapper Pattern comes into play. According to Fowler, the DataMapper is a “a layer of Mappers that move data between object and a database while keeping them independent of each other and mapper itself”. The core motivation behind the pattern is separation of concerns; an abstraction between the data, the store, and entity objects themselves. By using a DataMapper we can associate the name/value pair scenario described above with our domain objects, providing structure to our model. This is important because we go from an untyped hash-map model to tangible objects (POCO) that are typed, thus allowing us the benefits of developer friendly features of our IDE, such as ‘intellisense’ and compile time type safety.
Implemented within an API, you get something that looks like the following:
Think of it as a minimalistic ORM. In this case such concerns as caching and transactions are the responsibility of the underlying data-store service. As a consumer we are only concerned with making a request for data, getting a result set, and mapping it to an entity or domain object. In the diagram above, there are actually three different patterns working to create the whole: the command pattern, the repository pattern, and the data mapper pattern. All three comprise the entire API.
A Simple DataMapper Implementation:
This implementation of a DataMapper really does just three things: a) it reads through an XML stream, b) it maps to the values of specific XML attributes to an associated property, and c) packages the results into either a collection or a single object of the Type provided defined in the generic argument:
Map maps a single result to a single object. MapAll maps multiple results to a list of objects, and Read is a private method that extracts all key value pairs (XML attributes and there associative values) to a dictionary.
All Read does is extract out the XML attribute names and there associative data and store them in a dictionary. The dictionary is then used by the Map and MapAll methods along with a System.Attribute class to map the values directly to the correct properties:
In the code snippet above, after the Read method has extracted the data for mapping we loop over each of the objects properties looking for a FieldMappingAttribute on each of the properties we have flagged for mapping. If a match is found the relevant data is mapped directly to the object’s property and then the object is returned.
MapAll does essentially the same as Map, but assumes that the stream contains more than one object definition and then returns the resulting collection.
A couple of things to note here, the code a) needs a little refactoring to improve performance and extensibility, b) we are assuming a relatively flat xml file streamed here, no nesting of elements for example and c) we are only supporting properties of the Type System.String . The simplicity is purposeful…when I need it I’ll add it.
One last thing to discuss is implementation of FieldMappingAttribute and its usage. FieldMappingAttribute is a simple class deriving from System.Attribute that allows us to decorate properties on our entity classes with attributes that define our mapping schema:
The usage is pretty simple and should be familiar to most .NET developers: