Entity Core Insert Adding Duplicate Rows - entity-framework

Question

I'm trying to inserts an object and it's coming from a JSON file. The object has relationships which I've configured in EF Core and show in the database properly.
Unfortunately some of the data has duplicate entries in it but the Save action only happens at the end - so instead of getting a single child I get two child object when only one should exist.
JArray obj2 = (JArray)result.SelectToken("resistances");
List<CardResistance> pCardResistances = new List<CardResistance>();
foreach (var result2 in obj2)
{
//Search for Resistance Value object and create if not exists
Resistance ResistanceObj = ctx.Resistances.SingleOrDefault(m => m.ResistanceValue.Equals((string)result2["value"]))
?? new Resistance
{
ResistanceValue = (string)result2["value"],
LastUpdateDate = DateTime.Now
};
pCardResistances.Add(
new PokemonCardResistance
{
PokemonCard = PokemonCardObj,
Resistance = ResistanceObj
}
);
}
CardObj.CardResistances = pCardResistances;
...
ctx.AddOrUpdate and ctx.SaveChanges occur later in the code.
If you have JSON with the:
"resistances": [
{
"type": "Value1",
"value": "-30"
},
{
"type": "Value1",
"value": "-30"
}
]
In it what's happening is both rows get inserted for the entity (caused as the Save happens right at the end of all of the object data being populated. How can I save just the child object after each loop so it won't insert this data twice?

Answer 1

Your problem is to do with how you are checking for existence:
List<CardResistance> pCardResistances = new List<CardResistance>();
foreach (var result2 in obj2)
{
//Search for Resistance Value object and create if not exists.
//This will do a DB hit each time - if it doesn't exist and you have two
//items of the same value to add, then you will get that it does not exist
//each time.
Resistance ResistanceObj = ctx.Resistances.SingleOrDefault(m =>
m.ResistanceValue.Equals((string)result2["value"]))
?? new Resistance
{
ResistanceValue = (string)result2["value"],
LastUpdateDate = DateTime.Now
};
//One way to fix this would be to check that there is not already an item in this
//list that has this value set. And only either add/overwrite if that is the case.
pCardResistances.Add(
new PokemonCardResistance
{
PokemonCard = PokemonCardObj,
Resistance = ResistanceObj
}
);
}
CardObj.CardResistances = pCardResistances;
The other thing to note is more of a question as to what this should do exactly. Getting two updates in the same JSON for the same entity like this is a bit of an odd approach - is this valid?
If it is valid, then which one should 'win' (first or last in the json) and you may then be better placed altering the collection that you loop around (obj2 in your code - less said about your naming conventions the better...).