Questions on DAO design

This is the first time I used DAO design.
I have a machine class and each machine has a few parts.
How should I design the DAO? Especially regarding the inner parts.
Should I create a DAO for machine and for part and put the PartDAO as an atribute of MachineDAO so that I can populate the Machine's parts with the help of PartDAOOr is it better to retrieve the parts directly when retrieving machine by JOINING the 2 tables (inside MachineDAO).
Or are there other best practices regarding DAO design?
Please help me,
Thx in advance,
David Limanus

Right sorry. When you think from the user of the DAO
yes, one DAO should deal with one object. Then when
you start implementing the DAO, You will try to
reduce the overall number of tables. I don't agree - the number of tables should be designed according to the data's needs and normalization rules, and the objects according to the problem's needs. Then you map one to the other. I don't understand what you mean by "reduce the overall number of tables".
But its true
that I may have 3 different DAOs that access the same
table. Meaning that one may access the table directly and others JOIN to it? Yes.
Also, that I have one DAO that uses multiple tables.Meaning JOINs? Yes.
Actually what I tried was to handle one table from
one class file. That didn't really work
either because of the joins and subqueries.
Inefficient by design: suffers from the (n+1)query
problem.Could you explain that problem? It seems to work
well for me and is easily managed.If you measure the performance and find that it meets your service level agreement, then it "works". But it depends on the number of Parts a Machine has, right? If you do one query for each Part, that means one network roundtrip for each one. As the number of Parts gets large, so does the number of network roundtrips. Since network latency is one of the biggest bottlenecks in your app, the more of them you do, the worse your performance.
A Machine that requires a small number of Parts might perform fine, but if the number increases you might find that performance is unacceptable. Maybe yours is "working well" because your Machines are simple.
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valooCK wrote:
NANDA wrote:
Hi,
what is the advantages of DAO Design pattern over other design patterns?
Can anybody tel methe answer is none!
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you may occasionary use it as a collection of functionalities or utilities, but thou shalt never use in as a pattern in oop!There are as-yet undiscovered tribes living in the Amazon, who knew you were going to say that
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That is exactly what I am trying to do. I am writing it all myself to get a better understanding of it.
Please bear with me, because there are some things I dont understand in your previous answer.
1) Each BO validates his corresponding DTO and exposes operations to persist that DTO. Each DTO will be persisted in the database via his corresponding DAO. So this would be:
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this.validate(addressDTO);
this.addressDAO.save(addressDTO);
Am I viewing it from the right side now?
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Edited by: Dat Triet Banh on Jul 4, 2011 4:47 AM

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or
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Best Regards
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>
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>
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>
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>
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--mark

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