Dapper nested list query - list

Question

I have 3 POCO classes as below;
Continent has many ContinentPart
Continent has many Country
Country has many City
I want to get Continens with ContinentParts, Countries and Cities
using (IDbConnection db = new SqlConnection(_conf["ConnectionStrings:WorkConStr"]))
{
string query = #"SELECT * FROM Continent as c
LEFT JOIN ContinentPart as cp ON c.ContinentId=cp.ContinentId
LEFT JOIN Country as co ON c.ContinentId=co.ContinentId
LEFT JOIN City ci ON co.CountryId=ci.CountryId
ORDER BY c.RecDate DESC";
var continentDictionary = new Dictionary<int, Continent>();
var list = db.Query<Continent, ContinentPart, Country, City, Continent>(
query,
map: (continent, continentPart, country, city) =>
{
Continent m;
if (!continentDictionary.TryGetValue(continent.ContinentId, out m))
{
continentDictionary.Add(continent.ContinentId, m = continent);
}
if (m.ContinentParts == null)
m.ContinentParts = new List<ContinentPart>();
m.ContinentParts.Add(continentPart);
if (m.Countries == null)
m.Countries = new List<Country>();
m.Countries.Add(country);
foreach (var h in m.Countries)
{
if (h.Cities == null)
h.Cities = new List<City>();
h.Cities.Add(city);
}
return m;
},
param: new { },
splitOn: "")
.Distinct()
.ToList();
return list;
}

Answer 1

You have two options here. You can use Multiple Resultsets feature:
https://medium.com/dapper-net/handling-multiple-resultsets-4b108a8c5172
Or, another option, better in my opinion, is to return the joined result as a JSON, for example with a query like the following:
select
continents.continent_id,
continents.continent,
countries.country_id,
countries.country,
cities.city_id,
cities.city
from
dbo.Continent continents
inner join
dbo.Country countries on continents.continent_id = countries.continent_id
inner join
dbo.City cities on countries.country_id = cities.country_id
for
json auto
that returns a JSON like this:
{
"continent_id": 1,
"continent": "Africa",
"countries":
[
{
"country_id": 1,
"country": "Nigeria",
"cities":
[
{
"city_id": 1,
"city": "Lagos"
}, {
"city_id": 2,
"city": "Abuja"
}
]
}
]
}
and that can then be turned into a complex object using Custom Type Handling:
https://medium.com/dapper-net/custom-type-handling-4b447b97c620
Links are to articles I've wrote on the subject.