var results = from animal in context.Set<Animal>()
    where animal.Details != null
    select animal;

Unhandled exception. System.InvalidOperationException: The LINQ expression 'DbSet<Animal>()
    .Where(animal => animal.Details != null)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.
   at MongoDB.EntityFrameworkCore.Query.MongoQueryCompilationContext.CreateQueryExecutor[TResult](Expression originalQuery)
   at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass11_0`1.<ExecuteCore>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteCore[TResult](Expression query, Boolean async, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.GetEnumerator()
   at Program.<Main>$(String[] args) in /Users/arthur.vickers/code/OnYourOwnNow/OnYourOwnNow/Program.cs:line 29

using var context = new SomeDbContext();

context.Database.EnsureDeleted();

context.Add(
        new Animal { Id = 1 });

context.Add(
        new AnimalDetails
        {
            Id = 1,
            AnimalId = 1,
            BoolField = true
        });

context.SaveChanges();

var results = from animal in context.Set<Animal>()
    where animal.Details != null
    select animal;

foreach (var result in results)
{
    Console.WriteLine($"Found {result}");
}


public class SomeDbContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) 
        => optionsBuilder
            .UseMongoDB("mongodb://localhost:27017", "SomeDB")
            .LogTo(Console.WriteLine, LogLevel.Information);

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Animal>();
        modelBuilder.Entity<AnimalDetails>();
    }
}


public class Animal
{
    public int Id { get; set; }
    public AnimalDetails? Details { get; set; }
}

public class AnimalDetails
{
    public int Id { get; set; }
    public int? AnimalId { get; set; }

    public bool BoolField { get; set; }
}