var result = postCollection
    .AsQueryable()
    .Select(p => new
    {
        Id = p.Id,
        Comments = p.Comments.Where(c => c.Text.Contains("test"))
    })
    .ToList();

Message: System.NotSupportedException : Contains of type System.String is not supported in the expression tree {document}{$c.Text}.Contains("test"). Stack Trace: AggregateLanguageTranslator.TranslateMethodCall(MethodCallExpression node) AggregateLanguageTranslator.TranslateValue(Expression node) AggregateLanguageTranslator.TranslateWhere(WhereExpression node) AggregateLanguageTranslator.TranslateValue(Expression node) AggregateLanguageTranslator.TranslatePipeline(PipelineExpression node) AggregateLanguageTranslator.TranslateValue(Expression node) AggregateLanguageTranslator.TranslateMapping(ProjectionMapping mapping) AggregateLanguageTranslator.TranslateNew(NewExpression node) AggregateLanguageTranslator.TranslateValue(Expression node) AggregateLanguageTranslator.Translate(Expression node, ExpressionTranslationOptions translationOptions) QueryableTranslator.TranslateProjectValue(Expression selector) QueryableTranslator.TranslateSelect(SelectExpression node) QueryableTranslator.Translate(Expression node) QueryableTranslator.TranslatePipeline(PipelineExpression node) QueryableTranslator.Translate(Expression node) QueryableTranslator.Translate(Expression node, IBsonSerializerRegistry serializerRegistry, ExpressionTranslationOptions translationOptions) MongoQueryProviderImpl`1.Translate(Expression expression) MongoQueryProviderImpl`1.Execute(Expression expression) MongoQueryableImpl`2.GetEnumerator() List`1.ctor(IEnumerable`1 collection) Enumerable.ToList[TSource](IEnumerable`1 source)