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Inheritance Concept&Association Concept

Inheritance Mapping
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Table Per Class
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-> We select this inheritance strategy when we want to save the data of all classes heirarchy into a single table of database.
-> In this we use an extra column to identify, For this we use discriminator column in the table.
-> And we configure <discriminator> tag in hbm file after closing <id> tag And <subclass> tag for child class.

Table Per Concrete Class
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-> This inheritance strategy is selected when there is a need to store each concrete class object of inheritance in separate table of database.
-> In hbm file we need to configure <union-subclass> tag under <class> tag.

Table Per SubClass
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-> We choose this inheritance strategy if we want to map a super class and its subclass to its own tables of database.
-> A subclass table has primary key and also acts as foreign key to get the relation with super class table.
-> Each subclass table has foreign key column. So to tell the hibernate about foreign key column we need to use a <key> tag.
-> In hbm file we need to configure <joined-subclass> tag under <class> tag.

Associations/Relationships
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One To Many Relationship
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1. In parent class we need to take a reference variable of type Collection.
2. In parrent class mapping, we need to map the Collection also.
3. In Client code, we need to add objects of child class to Collection object
   and then we need to set Collection object to Parrent object.

Many To One Relationship
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1. In Many To One Relationship, we need a reference variable of type Parent class
   in Child class.
2. In Child class mapping, we need to map parrent class also.

Many To Many Relationship
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1. It is the combination of One To Many and its reverse also One To Many.
2. In This, both classes are parent classes and also child classes. So we need a reference variable of type Collection in both the classes.
3. In this one extra table is created for join table bcoz database doesn't support Many to Many relationship.

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