Today we continue the discussion  on DBPowder. We noted earlier the process involved in the code generation, specifically given a conceptual model, how to map relational and object model. When EER is modeled using a directed graph, the starting entity is called the pivot entity. Connectivity on the edge is defined along with that of the corresponding relationship.Hierarchy is represented by directional edges from parent entity to child entity. When representing relationship, if there are attributes to the relationship, then they belong to the connected node to which the connectivity is many.If there are more than one path from the starting point to E, E has to fill all the conditions that correspond to the incoming edges. If this is not preferable, ObjectView can reduce the number of incoming edges by introducing another corresponded node and modifying one of the paths to use this node.  When grouping of nodes within a sub-graph, we form with those where the connectivity of the corresponding relationship is one. ObjectView arranges the class definitions generated from the grouped node  using the keywords structured, literal and interface. The structured literal is defined as a fixed number of  literals and the interface is defined as the abstract behavior of an object type. A structured literal can be used as a user defined literal. The interface is independent of the directed graph and applied to the group node.Multiple interfaces are possible for a grouped node in which case all the operations definitions are defined in the corresponding class.
In the example we took earlier with users and hosts, the application logic has to start from the user and hence the pivot entity is chosen. the node register and host are grouped.  The generated classes are user oriented. It's also possible to use host as pivot entity, in which case the sub-classes of the user are not required.
By using different ObjectView pivot entity, we can form the attributes from the grouped node. A practical generation of source code and their structure would involve code generation that corresponds to active records that has attribute values, setters/getters methods for data classes, and persistent classes for the wrappers of the data wrapper classes. Together with the active record and the logic classes, data persistence can proceed with the application codes defined by the developer and the call persistence methods on the session class that translate to SQL DDL. Developers of .Net framework can quickly associate these pieces of generated code to the templates defined from say the EDM.

Today we continue our discussion on DBPowder. We briefly reviewed the data models  - namely the EER model, the Object model and the relational model.  We now see how to use the EER model for simple correspondences and ORM processes. When the developer describes the EER model and the DBPowder generates the corresponding spc object model and the rs relational model, it follows this process:
1) In eer, A surrogate key is added to each entity where primary key may have been omitted.
2) Each table and an attribute in rs is generated with one to one correspondence to an entity and an attribute in EER. The relationship is used to add foreign keys to either side of the tables.  Hierarchies are denoted later.
3) For each table, an attribute and a foreign key in rs, a class, attribute and relationship in spc is generated. Each relationship is converted into a bidirectional one by adding an inverse relationship. The cardinality is also added by looking it up in the EER.
The hierarchies were reserved for after step 3 because they are not directly mapped to the rs. Instead three methods are used.
1) Single Relational Inheritance - all of the classes in the a generalization hierarchy are mapped onto one table.
2) Class Relational Inheritance - each class in the hierarchy is mapped one to one into one table.
3) Concrete Class Relational Inheritance - each concrete class in a generalization hierarchy is mapped one to one into one table.
In the example describing user generalization earlier, SR was used.
The reg_date attribute of register entity proves tricky because a user has a many to many relationship to a host, and a SR relationship between persistent classes of User and Host cannot handle this attribute reg_date because reg_date is unidirectional.
For complex correspondences DBPowder introduces ObjectView. The process now continues as :
4) The Object View descriptions are denoted first:
    1) specifications of entities and relationships in the EER model use a directed graph.
     2) Nodes are grouped within a subgraph of the directed graph by combining the nodes which have a relationships of one-connectivity.
      3) class definitions are then generated from the group nodes using structured literal and interface.

Today we will continue our discussion on DBPowder. We will look into the conceptual model introduced by this framework. This paper proposes the following data models:
1) Object Model : This object is based on the subset of ODMG 3.0 standard, with some Java-based extensions. An object is called persistent  if all property values are saved into the storage in order they may be retrieved later. Let us take an example with a class User that has an attribute user_name and has a relationship register to another class Register. The Register class also has a relationship user to the class User. Therefore the two classes have a bidirectional relationship. The cardinality is 1:n  A class AdmUser is a sub class of the class User and the class User has a superclass of AdmUser and a class GuestUser
2) RelationalModel: The relational model has a table user and an attribute user_name and primary key user_id. A table register has a foreign key user_id to the table user.
3) EER model in DBPowder : The EER model describes the primitives such as attributes, entity, relationship, connectivity etc. The notations used are : e defines a fact,  e has one or more attributes A1, A2 .. An. Entity E comprises of  a set of facts | the entire attributes. Relationships are expressed as connectivity which represents cardinality.The relationship among three or more entities is described by another entity E' and connecting E' to each of the entities E1, E2, E3.
In the example taken earlier an entity user p has an attribute user_name q and a relationship r to an entity register. r has a connectivity 1:n. For entity user, the occurrence of entity register  is mandatory , a constraint represented by s. The entities of user, adm_user and guest_user form a hierarchy with user as the base. Thus the EER can be used to generate both the relational tables as well as the simple correspondences. 

Today we continue reading the paper on DBPowder. We brought up the mention that Hibernate allows multiple complex correspondence with HBM files. Using Hibernate Annotations which is a supplemental tool, the developer can describe classes with annotations but they too don't improve the Hibernate.
ADO.Net entity model is another popular method which has a conceptual schema description language, a store schema description language and a mapping specification language - all represented in xml. As part of MSL, Melnik proposed a method with bidirectional views which describe a set of constraints between a group of tables and persistent classes. The compiler compiles the constraints to generate the query views and update views for complex correspondences.
Msquare-ORMsquare is another method bur it requires a relational schema and persistent classes in advancey
A relational view extends the original table but views and persistent classes  still need to be described to utilize ORM moreover when data manipulations are performed on the views, they may not be properly reflected on the original table. Also views have to be unfolded to original tables with complex queries causing poorer performance. For object oriented software design, uml is popular.
In this paper, while the authors adopted an EER model to clarify the issues, a design in the EER model can be easily converted into that by the UML Class diagram. This enables simple to complex correspondence comparison because simple uses EER and complex uses ObjectView, in addition to EER.
#codingexercise


GetOddNumberRangePowerTenthRootPowerTwenty(Double [] A)


{


if (A == null) return 0;


Return A.OddNumberPowerTenthRootPowerTwenty();


}

Today we continue reading the paper on DBPowder. We were reviewing related technologies Today we will continue some more discussion. Hibernate is mentioned as an  ORM that handles complex correspondence. Here developers describe mappings between the persistent classes and the relational schema in hbm files. This file is used to generate code and table. However, each time the hbm needs to be updwted, it is pretty disruptive. Even with annotations the expressibility of the mapping isn't improved. While we talk about ORM framework, we don't talk about the mark and sweep required for Graph API. Those we read in distributed computing. Here we read the paper for improvements to the mapping framework. Also we were hinting at writing Graph API for Active Directory objects CRUD. That cannot work unless AD objects CRUD works via LDAP protocol. We need proper credentials and reserved OU. And even so, will most likely require a ticketing framework for changes to be captured in audit trail.

Today we continue reading the paper introducing DBPowder as we did in the previous post. We said that for complex correspondences, the developer needs to specify the complete correspondence among persistent classes and tables. which is not always possible because they are subject to change. With DBPowder, simple and complex correspondences are supported with a collaboration of a conceptual model and a navigational data usage description. During the initial rapid development, developers describe an Extended Entity Relationship model and DBPowder generates schema and classes for this model. During the latter spiral development, DBPowder introduces ObjectView, a graph based object description form over the EER model. The developers can refine their EER models and add ObjectView. and then DBPowder refines the existing relational schema and persistent classes and adds persistent classes for ObjectView.
In the discussion on related works, the authors mention ActiveRecord which is a one to one mapping between table and attributes - attributes of ActiveRecord are defined with those of the table. The advantage of this approach is that its simple but doesn;t work for complex correspondences.
Another example is the conceptual model eg. ER model as the basis such as WebML and Enterprise Objects Framework. Here schema and classes are generated from the conceptual model.
A third approach was introduced by Thiran et al which uses wrapper based ORM and applies the eight transformation operators one after the other to a target relational schema.  The descriptive power is limited within these operators.
#codingexercise


GetOddNumberRangePowerSeventhRootPowerTwenty (Double [] A)


{


if (A == null) return 0;


Return A.OddNumberPowerSeventhRootPowerTwenty();


}

Today we will start reading another research paper. This one is titled : DBPowder : A flexible object relational mapping framework based on a conceptual model. written by Murakami, Amagasa, Kitagawa et al. Object Relational mapping framework are well known to web application developers for the convenience they provide in programming data objects and persisting to a database. The relational schema is abstracted by an object graph that enables seamless data persistence. The authors begin by recognizing that there are many such ORM frameworks and most of them have to compromise two contradictory requirements, 1) that is the support of persistence classes that are directly mapped to relational table. and 2) the support of complicated compositions of base classes which are required by the application. These are two strong requirements and they become clearer as the application evolves. Initially a one to one mapping between each persistent class and their respective table is required and as the development proceeds, more complicated correspondences between persistent classes and relational tables are needed as the development proceeds. Foytr this reason, it is desirable for an ORM framework to be flexible enough between these two ends of the spectrum at different development stages.
In this paper, the authors do that by proposing a framework called DBPowder that addresses the difficulty in handling simple and complex correspondences.  DBPowder supports direct mapping to tables with Extended Entity Relationship Model and it supports complicated compositions with ObjectView, a graph based object description form over the EER model.  The EER model and the ObjctView together provide the flexibility that is required in different development stages.
Through out this paper the authors use the term simple correspondence and complex correspondence for 1) and 2) mentioned earlier. They refer to the classes in ORM as a) persistence classes that deal with persistent data in an object oriented language. b) relational schema to manage the persistent data in the RDB. and c) data conversion between object states and RDB queries and responses.
If we take the example of users and hosts, that have a many to many relationship as captured in a third join table, then we can have a simple correspondence with more complicated mappings possible by defining the mappings by hand.