Comparison of EF query mapping approaches.

IQueryableSql.sql

--Microsoft.EntityFrameworkCore.Database.Command:Information: Executed DbCommand (176ms) [Parameters=[], CommandType='Text', CommandTimeout='30']

SELECT CASE
    WHEN EXISTS (
        SELECT 1
        FROM [Customers] AS [x])
    THEN CAST(1 AS BIT) ELSE CAST(0 AS BIT)
END

--Microsoft.EntityFrameworkCore.Database.Command:Information: Executed DbCommand (185ms) [Parameters=[], CommandType='Text', CommandTimeout='30']

SELECT COUNT(*)
FROM [Customers] AS [x]

--Microsoft.EntityFrameworkCore.Database.Command:Information: Executed DbCommand (77ms) [Parameters=[@__p_0='?' (DbType = 
Int32), @__p_1='?' (DbType = Int32)], CommandType='Text', CommandTimeout='30']

SELECT [x].[Id], [x].[Email], [x].[FirstName], [x].[LastName]
FROM [Customers] AS [x]
ORDER BY [x].[LastName], [x].[FirstName] DESC, [x].[Id]
OFFSET @__p_0 ROWS FETCH NEXT @__p_1 ROWS ONLY

--Microsoft.EntityFrameworkCore.Database.Command:Information: Executed DbCommand (86ms) [Parameters=[@__p_0='?' (DbType = Int32), @__p_1='?' (DbType = Int32)], CommandType='Text', CommandTimeout='30']

SELECT [t].[LastName], [t].[FirstName], [t].[Id], [x.School].[Name] AS [Item1], [x.CustomerSchools].[SchoolId] AS [Item2], [x.CustomerSchools].[CustomerId]
FROM [CustomerSchool] AS [x.CustomerSchools]
INNER JOIN [Schools] AS [x.School] ON [x.CustomerSchools].[SchoolId] = [x.School].[Id]
INNER JOIN (
    SELECT [x0].[LastName], [x0].[FirstName], [x0].[Id]
    FROM [Customers] AS [x0]
    ORDER BY [x0].[LastName], [x0].[FirstName] DESC, [x0].[Id]
    OFFSET @__p_0 ROWS FETCH NEXT @__p_1 ROWS ONLY
) AS [t] ON [x.CustomerSchools].[CustomerId] = [t].[Id]
ORDER BY [t].[LastName], [t].[FirstName] DESC, [t].[Id]

ProjectToResult.sql

--Microsoft.EntityFrameworkCore.Database.Command:Information: Executed DbCommand (182ms) [Parameters=[], CommandType='Text', CommandTimeout='30']

SELECT CASE
    WHEN EXISTS (
        SELECT 1
        FROM [Customers] AS [x])
    THEN CAST(1 AS BIT) ELSE CAST(0 AS BIT)
END

--Microsoft.EntityFrameworkCore.Database.Command:Information: Executed DbCommand (155ms) [Parameters=[], CommandType='Text', CommandTimeout='30']

SELECT COUNT(*)
FROM [Customers] AS [x]

--Microsoft.EntityFrameworkCore.Database.Command:Information: Executed DbCommand (124ms) [Parameters=[@__p_0='?' (DbType = Int32), @__p_1='?' (DbType = Int32)], CommandType='Text', CommandTimeout='30']

SELECT [x].[Id], [x].[CreateDateUtc], [x].[Email], [x].[FirstName], [x].[LandLine], [x].[LastName], [x].[Title], [x].[UpdateDateUtc]
FROM [Customers] AS [x]
ORDER BY [x].[LastName], [x].[FirstName] DESC, [x].[Id]
OFFSET @__p_0 ROWS FETCH NEXT @__p_1 ROWS ONLY

--Microsoft.EntityFrameworkCore.Database.Command:Information: Executed DbCommand (150ms) [Parameters=[@__p_0='?' (DbType = Int32), @__p_1='?' (DbType = Int32)], CommandType='Text', CommandTimeout='30']

SELECT [x.CustomerSchools].[CustomerId], [x.CustomerSchools].[SchoolId], [c.School].[Id], [c.School].[Address], [c.School].[CreateDateUtc], [c.School].[CreateUser], [c.School].[DPID], [c.School].[DeniedReason], [c.School].[Name], [c.School].[OldId], [c.School].[Status], [c.School].[SuburbId], [c.School].[UpdateDateUtc], [c.School].[UpdateUser]
FROM [CustomerSchool] AS [x.CustomerSchools]
INNER JOIN [Schools] AS [c.School] ON [x.CustomerSchools].[SchoolId] = [c.School].[Id]
INNER JOIN (
    SELECT [x0].[Id], [x0].[LastName], [x0].[FirstName]
    FROM [Customers] AS [x0]
    ORDER BY [x0].[LastName], [x0].[FirstName] DESC, [x0].[Id]
    OFFSET @__p_0 ROWS FETCH NEXT @__p_1 ROWS ONLY
) AS [t] ON [x.CustomerSchools].[CustomerId] = [t].[Id]
ORDER BY [t].[LastName], [t].[FirstName] DESC, [t].[Id]

Query.cs

    public static class MappingExtensions
    {
        public static IQueryable<Index.Model> MapCollectionTo(this IQueryable<Customer> customers)
        {
            return customers.Select(customer => new Index.Model
            {
                Id = customer.Id,
                Email = customer.Email,
                FirstName = customer.FirstName,
                LastName = customer.LastName,
                Schools = customer.CustomerSchools.Select(x => new Tuple<string, Guid>(x.School.Name, x.School.Id)).ToArray()
            });
        }
    }

    /// <summary>
    /// Provides named configuration for maps.
    /// </summary>
    public class MappingProfile : Profile
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="MappingProfile"/> class.
        /// </summary>
        public MappingProfile()
        {
            this.CreateMap<Customer, Index.Model>().ConvertUsing(source => Map(source));
        }

        /// <summary>
        /// Maps the <see cref="Customer"/> to an <see cref="Index.Model"/>.
        /// </summary>
        /// <param name="customer">The customer.</param>
        /// <returns>The <see cref="Index.Model"/>.</returns>
        public static Index.Model Map(Customer customer)
        {
            return new Index.Model
            {
                Id = customer.Id,
                Email = customer.Email,
                FirstName = customer.FirstName,
                LastName = customer.LastName,
                Schools = customer.CustomerSchools.Select(x => new Tuple<string, Guid>(x.School.Name, x.School.Id))
            };
        }
    }

Interesting things to note.

• The SQL generated via AutoMappers ProjectTo<T>(IQueryable) selects extra properties
• If the ToArray() call is removed, EF Core will generate an additional SQL query for each school in the IQueryable extension method, with ProjectTo the SQL remains the same

It would be nice if I could figure out a way to avoid multiple queries but EF Core does this by design for navigation properties in 2.x