Typically we use an OO model in the applicaiton but a non-OO (relational) model in the database. IMPEDENCE MISMATCH says that the models are different enough to cause problems. eg have to deconstruct objects to store them, object relaships r complex
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Why is ORM it not that bigger deal?
Most object relationships map quite well to foreign keys. Methods can be implemented as stored procedures.Database access APIs like JDBC simplifiy some of the differences.
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Alternatives?
Serialisation - save to file (cant query, no transactions) or Object DBMS
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Why are relational/SQL DBMS still best?q
Mature, well understood, well defined standards
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Object Relational Mapping (ORM)
Category of libraries/frameworks for persistence of OO data in relational databases. On the fly conversion of domain objects to conform them to table struct of DB. Lets us query our data using the OO model. We still have a reltnal DB,bt treat it OO
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Why ORM
can be very productive, do a lot of caching which can speed up processing.
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Object Mapping
We need a way to map the applications OO domain classes to and from the databases relational tabes. Mapping lets us control how this happens.
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Generating mappings:
Top Down (Start from the OO model - not so good for large systems). Bottom Up (Makes sense if database already exists). Hybrid - Start with both domain model and databse. Create each independently, use mapping to glue them together.
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Ad and Dis of ORM frameworks
Can be very productive, improved maintentance, Caching, Widely used. Dis = over complex for small systems. Errors hard to deal with for non expereinced.
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Card 2
Front
Why is ORM it not that bigger deal?
Back
Most object relationships map quite well to foreign keys. Methods can be implemented as stored procedures.Database access APIs like JDBC simplifiy some of the differences.
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