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Querying relative information with RavenDB
time to read 2 min | 295 words
Originally posted at 12/3/2010
Let us say that we have the following document:
{
"Name": "Ayende",
"LastScore": 239.2,
"MaxScore": 392.6
}
And we want to find all documents whose last score is within a certain range from their max score. Note that for different queries, the range that I can use may be different.
RavenDB doesn’t offer the option of doing computation in the where clause. Mostly, that is because such computations are going to perform badly unless special care is taken to avoid that. Instead, we are going to create a computed field in the index.
First, we define:
from u in docs.Users where u.MaxScore select new { Score = u.LastScore / u.MaxScore }
This computed field now allows us to query on it very easily. Moreover, when we query, we are still querying over pre-computed data, which is going to blindingly fast.
Comments
I don't get it.
How can you do a 'where' on a float? Or is this some RavenDB specific interpretation of Where?
note: my knowledge of RavenDB goes no further than what I picked up from this blog.
its a typeo no?
It looks like he's making sure MaxScore is not 0 or null, which would cause the division to blow up
He's also selecting the score, not the document. I don't know how you're supposed to get the document from the score.
I think he rushed the post a bit. :)
Nitpick: this is not querying relative information like the title says. This is transforming relative to non-relative and querying only absolute values.
@Rafal yes, but if the document changes, the index gets updated so the value gets recomputed, so it works the result is the same as if you queried relative information.
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