c# - MongoDB - How to model recursive relation (categories) -

i'm working on application have categories it's entries. each entry have 1 category. concern how model categories in order achieve cleanest code , design.

this how thinking implement category entity - approach 1:

public class category : entity {     public string name { get; set; }     public string desc { get; set; }     public int guiorder { get; set; }     public list<category> categories { get; set; } } 

in order achieve nested categories , instantiate them in way:

    var cat1 = new category()     {         name = "bikes",          desc = "mountain, city, you-name-it have bikes.",          guiorder = 1,         categories = new list<category>()         {             new category()             {                 name = "sub 1",                 desc = "sub 1 description.",                 guiorder = 1,                 //id =              }         }     }; 

this how document in mongodb. can see in "sub 1", id commented out, because entity definition nested category mongodb not generate key, i'll have explicitly in application, like:

subcat1.id = new bsonobjectidgenerator().generateid(categoryrepository.collection, subcat1).tostring(); 

and since don't have reference "subdocument1" in object initializer, have done later, probalz recursively before submitting object. seems bit awkward.

as alternative, following entity definition (approach 2):

public class category : entity {     public string name { get; set; }     public string desc { get; set; }     public int guiorder { get; set; }     public category parent { get; set; } } 

since categories flattened relational approach , don't here mongo insert whole objects, not objectids only:

{         "_id" : objectid("53413657e42c3c16582cda70"),         "name" : "bikes",         "desc" : "mountain, city, you-name-it have bikes.",         "parent" : null } {         "_id" : objectid("53413657e42c3c16582cda72"),         "name" : "sub 1",         "desc" : "sub 1 description.",         "parent" : {                 "_id" : objectid("53413657e42c3c16582cda70"),                 "name" : "bikes",                 "desc" : "mountain, city, you-name-it have bikes.",                 "parent" : null         } } {         "_id" : objectid("53413657e42c3c16582cda73"),         "name" : "sub 2",         "desc" : "sub 2 description.",         "parent" : {                 "_id" : objectid("53413657e42c3c16582cda70"),                 "name" : "bikes",                 "desc" : "mountain, city, you-name-it have bikes.",                 "parent" : null         } } 

so concerns are: approach 1 - how seek through categories efficiently (they nested @ 4 levels)? let's feature: "show in category"... assigning obejctid on application level bad practice? doubt need sharding ever...

or approach 2 if "main" category changes? - i'd have change on several places. deep hirearchy can messy , repetitive.

i appreciate feedback on solution better , proposed improvements. can expect problems 1 or another? better picture - these categories not changing (i might not implement ui manipulation).