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PODealing with nested aggregates in DDD
 

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  1. PODealing with nested aggregates in DDD
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    <p>I'm just getting started in DDD, and I'm having some trouble figuring out how to accommodate the relational nature of my data. I have what I believe would be considered my aggregate root, but the aggregate also has an aggregate of its own. Not wanting to violate the Law of Demeter, I'm wondering if I'm thinking about this wrong and am hoping that some DDD expert can offer some insight.</p> <p>My aggregate root is my <code>Account</code> object, which has an aggregate of numerous <code>AccountElement</code> entities, which are themselves logical groupings of individual <code>ProductComponent</code> entities.</p> <p>An <code>AccountElement</code> outside of the context of an <code>Account</code> has no meaning, so I'm comfortable with my conclusion that the <code>Account</code> object is my aggregate root, and I anticipate that entity having an aggregate <code>Elements</code> property. It's the <code>ProductComponent</code> collection that has me confused. This aggregate has no meaning outside of an <code>AccountElement</code>, and <em>really</em> has no meaning outside of an <code>Account</code>.</p> <p>I don't think I should be accessing individual <code>ProductComponent</code> objects by dotting my way to it, like:</p> <pre><code>var reference = account.Elements(0).ProductComponents(0).ReferenceCode; </code></pre> <p>But at the same time it doesn't make sense (from a domain perspective) to access a <code>ProductComponent</code> directly from an <code>Account</code> entity.</p> <p>I'm sure that this is all a little difficult to comprehend without knowledge of my domain, but I'm hoping it's enough to get some good feedback.</p>
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    1. USJosh Anderson
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      1. PODealing with nested aggregates in DDD
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    1. COIf you require this level of nesting objects to make your object model work, then I wouldn't worry too much about the Law of Demeter. See http://haacked.com/archive/2009/07/14/law-of-demeter-dot-counting.aspx
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      1. PODealing with nested aggregates in DDD
      1. USRobert Harvey
    2. COSee also http://www.dcmanges.com/blog/37
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      1. PODealing with nested aggregates in DDD
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    3. COThanks, Phil's article was helpful, and the question of how inextricably related the children of the children are to the aggregate root is really key here. Looking at that, it only makes sense to nest the entities. I'm more comfortable this is correct because after making this decision the direction of the code seems to be a lot more intuitive. Or in the spirit of DDD, I might be better off saying it is a truer abstraction of the ubiquitous language ;)
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      1. USJosh Anderson
 

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