Aggregating date data with entity framework grouped by day, month, qtr, year, etc - entity-framework

Question

I have a table that records activities. All activities have an activitydate. I want to count how many activities for a given period of time (day, month, qtr, etc.). I want to include all dates even those that have zero activities. I could do this in the Data Tier with a DateDimension table where the date table has a single column called day containing one row for each calendar day and a outer join, group by query:
DateDimension Table
| Day |
|1/1/2013 00:00:00 |
|1/1/2013 00:00:00 |
|1/1/2013 00:00:00 |
Query
SELECT CAST(Day AS DATE), COUNT() AS CountOfActivities
FROM DateDimension dd LEFT OUTER JOIN Activities a
ON CAST(dd.Day AS DATE) = CAST(a.ActivityDate AS DATE)
WHERE Day BETWEEN MyStartDate AND MyEndDate
GROUP BY CAST(Day AS DATE)
ORDER BY CAST(Day AS DATE)
I'm using EntityFramework so I'd like to execute this query using Linq. The DateDimension table has no business value residing in the database. It exists only to support these aggregate queries by providing a list of dates so I can ensure a row is returned if no activities exist for a given day.
I have the idea that I could manufacture a list of days in memory and weave them in to the results of a much simpler database query at runtime. By perhaps Concatenating the results from 2 IEnumerables - 1 from the in memory enemurable of dates and the other from the database results. How could I do that? Should I do that?

Answer 1

How about something like this:
Example date range:
var from = DateTime.Today.AddDays(-30);
var to = DateTime.Today;
Dictionary to hold your tally of activities per day:
var activityCounts = new Dictionary<DateTime, int>();
Seed with a zero count for each day in the range (this is equivalent to setting up your date dimensions table):
Enumerable.Range(0, (to - from).Days + 1)
.ToList()
.ForEach(x => activityCounts[from.AddDays(x)] = 0);
Add in the real activity counts for each day in the range:
context.Activities.Where(a => a.DateTime >= from && a.DateTime <= to)
.GroupBy(a => a.DateTime)
.ToList()
.ForEach(x => activityCounts[x.Key] = x.Count());
In this way, you only hit the database for the aggregation of activities for dates with activities. The padding out of the resultset with contiguous dates within the date range is then performed on the app server.
Just need to be careful how your dates are stored in the database. This code example is expecting to be able to match keys in the activity dictionary based on the the format of the calls to DateTime.Today. You will need to shape your dates in your database query accordingly.