one-to-many insert using dapper dot net

Question

I have seen here

How to insert an IEnumerable<T> collection with dapper-dot-net

how dapper is able to handle IEnumerable as an input param and dispatch multiple commands for each member of the collection.

In my case I have an IEnumerable<int> Categories and a int SurveyId and I want to insert this one-to-many relationship into a separate mapping table called SurveyCategories

Is there a LINQ extension that I could use to concatenate these Categories with the same SurveyId, similar to .Concat()?

Or should I loop through the collection and build up a new list of objects with SurveyId and CategoryId properties?

Answer 1

You could do one insert for the surveys, and then insert all the survey categories at once using the following linq query as the parameter:

var allSurveyCategories = surveys.SelectMany(s =>
 s.Categories.Select(c => new{SurveyId = s.SurveyId, CategoryId = c}));

• Pricey: I am confusing matters a bit by trying to adjust my example. I have updated my answer to give an example of what I am actually doing. Your answer looks better but I am still trying to understand how to implement it. Thanks
• Pricey: I have updated my answer based on your use of .SelectMany, its slightly less confusing.

Answer 2

Here is what I have done so far, I changed the categories slightly to an int array called CategoryIds just due to how its being used in my system, but I could have done survey.Categories.Select(c => c.Id).Zip(...

// insert using source data from form post matching a ISurvey interface, where .Id will be 0
var surveyId = conn.Query<int>("INSERT ... " + 
 "SELECT CAST(SCOPE_IDENTITY() AS INT)")
 .First();
 if(source.CategoryIds != null && source.CategoryIds.Count() > 0) {
 var surveyCategories = source.CategoryIds
 .Zip(
 Enumerable.Repeat<int>(
 surveyId,
 source.CategoryIds.Count()
 ),
 (c, s) => new { SurveyID = s, CategoryID = c }
 );
 conn.Execute(@"INSERT INTO [SurveyCategories] " + 
 "VALUES (@SurveyID, @CategoryID)", 
 surveyCategories);
 }

UPDATE: Here is my new approach using SelectMany based on Eren's answer, the use of Enumerable.Repeat(..) is a bit of a hack but this is the only way I have been able to do the same thing so far.

 ...
 var surveyCategories = source.CategoryIds.SelectMany(
 s => Enumerable.Repeat(surveyId, 1),
 (c, s) => new { SurveyID = s, CategoryID = c });
 ...

• Tilak: You cannot avoid looping. If number of elements are same, Zip is OK. Otherwise you need joins