Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <h3>Thoughts on DDD decisions...</h3> <p>It sounds like you want to be able to do this:</p> <pre><code>var people = session.Query&lt;Person&gt;() .FetchMany(x =&gt; x.CreatedOrders) .Where(x =&gt; x.Id == personId); </code></pre> <p>You should just flip the query around, like so...</p> <pre><code>var orders = session.Query&lt;OrderHeader&gt;() .Fetch(x =&gt; x.CreatedBy) .Where(x =&gt; x.CreatedBy.Id == personId); </code></pre> <p>There's more reasons than just DDD purity to keep <code>person.CreatedOrders</code> out of your model. What if you have one person that places 10 orders a day, every day of the year, year after year? The first query almost certainly loads more data than you actually need. The second query, the one that doesn't need a <code>person.CreatedOrders</code> collection at all, gives you more control over which orders to fetch and how many to fetch.</p> <pre><code>var orders = session.Query&lt;OrderHeader&gt;() .Fetch(x =&gt; x.CreatedBy) .Where(x =&gt; x.CreatedBy.Id == personId) .Where(x =&gt; x.OrderDate &gt;= DateTime.Now.AddYears(-1)) .Skip(100) .Take(50); </code></pre> <p>Generally, if you find you're violating a DDD principle, it's usually a good indicator that you're doing something wrong, and you usually don't have to look too far to find other supporting evidence.</p> <p>Every once in a while, though, you do have to bend the rules a bit, and I think that's acceptable. For example, I usually try to just map one side of each relationship - that's all I really need from a DDD perspective. When referencing other aggregate roots, that usually means just modeling the <code>many-to-one</code> side, and for references within an aggregrate root - from the root to its children, that usually means just modeling the <code>one-to-many</code> and leaving the <code>many-to-one</code> out of the model. That would mean leaving <code>OrderItem.OrderHeader</code> out of the model.</p> <p>What if you needed to find out how much of a particular product had been sold, but only for Orders that have been Submitted? The easiest way to do this would be like so:</p> <pre><code>var quantitySold = session.Query&lt;OrderItem&gt;() .Where(x =&gt; x.Product.Id == productId &amp;&amp; x.OrderHeader.Submitted) .Sum(x =&gt; x.Quantity); </code></pre> <p>So in this case, we need to model both the <code>one-to-many</code> <code>orderHeader.OrderItems</code> and also the <code>many-to-one</code> <code>orderItem.OrderHeader</code>. This is because it is most efficient for this SUM to access the relevant OrderItems directly, rather than first go through <code>OrderHeader</code> as DDD would dictate. I'm OK with that.</p> <h3>Left outer joins on un-mapped relationships</h3> <p>Ignoring the DDD part of the question, this LEFT OUTER JOIN can be simulated by combining the results of two separate queries - one to get the orders...</p> <pre><code>var orders = session.Query&lt;OrderHeader&gt;() .Fetch(x =&gt; x.CreatedBy); </code></pre> <p>... and another to get the people that have no orders:</p> <pre><code>var peopleWithNoOrders = session.Query&lt;Person&gt;() .Where(p =&gt; !session.Query&lt;OrderHeader&gt;().Any(o =&gt; o.CreatedBy == p)); </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POIs a Person an aggregate root?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USDaniel Schilling
    1. USDaniel Schilling
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POIs a Person an aggregate root?
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. CO@Hao, in response to your "I'm also surprised(in a good way), that referencing the OrderHeader from OrderItem violates DDD". Eric Evans is actually the authority on DDD (if there is an authority) - and I actually have not read his book. It's on my to-do list. The reason I say that modeling only one side of the relationship is better from a DDD perspective - it makes it absolutely clear which entity is in charge of the relationship. There's no possibility for ambiguity.
      singulars
      1. PO
      1. USDaniel Schilling
    2. COIf you map both sides of the relationship, then the `many-to-one` side, the child's reference to the parent, actually becomes the one in charge. Mapping just one side, when possible, more accurately represents the relationship. And generally you'll also find it's all you need. It also *forces* you to identify the aggregate roots and treat them as such, because for each relationship you have to decide "should I map the many-to-one or the one-to-many?" - this choice is determined primarily by which things are aggregate roots.
      singulars
      1. PO
      1. USDaniel Schilling
    3. COI've added something about the LEFT OUTER JOIN part of the question to my answer. See above.
      singulars
      1. PO
      1. USDaniel Schilling
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload