Hello Steven,
I am currently trying to post/create an entity in a collection via a relative binding path. Unfortunately, I always get an error saying

{"error":{"code":null,"message":"HTTP method 'POST' not allowed for this resource."}}

I have bound the root view to .../Account(1) from which I have a navigation property to the relevant collection. Currently I have trouble to post to collections via a navigation path.

As far as I understood from
http://docs.oasis-open.org/odata/odata/v4.01/csprd04/part1-protocol/odata-v4.01-csprd04-part1-protocol.html#sec_CreateanEntity

chapter 11.4.2 this should work according to my tries below.

Might you tell me if my path via the navigation property should work for post requests or what I am doing wrong?

Directly writing to the collection is fine, i.e. something like the following:

curl --user sap:sap -i -X POST -H 'Content-Type: application/json;charset=UTF-8;IEEE754Compatible=true' -d '

{"fkProfile":"1","idCode":null,"lc":"de","product_name":"A Product","brands":"A brand","energy_100g":2259,"carbohydrates_100g":60.08,"sugars_100g":10.1,"proteins_100g":24.03,"fat_100g":15.89,"saturated_fat_100g":10.0,"salt_100g":1.0}

' http://localhost:18080/odata4/svc/my_nutri_diary/UserDefinedProducts

and a GET request via a navigation property path (given below) also works, but a path via a navigation property does not work when using the path in a POST request. I.e. when going via the navigation property. Example:

curl --user sap:sap -i -X POST -H 'Content-Type: application/json;charset=UTF-8;IEEE754Compatible=true' -d '

{"fkProfile":"1","idCode":null,"lc":"de","product_name":"A Product","brands":"A brand","energy_100g":2259,"carbohydrates_100g":60.08,"sugars_100g":10.1,"proteins_100g":24.03,"fat_100g":15.89,"saturated_fat_100g":10.0,"salt_100g":1.0}

' http://localhost:18080/odata4/svc/my_nutri_diary/Account(1)/UserDefinedProducts_fKUserDefinedProductToAccount

Here you find the backend metadata. They show that UserDefinedProducts_fKUserDefinedProductToAccount is simply a navigaton path from Account(xxx) to the UserDefinedProducts collection.

https://ecubed-solutions.ddnss.de/odata4/svc/my_nutri_diary/$metadata
user: sap
password: sap

Thanks a lot for your help!

Note: I am currently trying to add data via a navigation property path because my frontend library sapui5 only caches and syncs data queried via a single context binding. Hence, to be consistent via all views in my app which bind to the odata model, I need to use only one absolute binding and have to query/update other contexts via relative binding paths from this single absolute path, to allow for all frontend views to maintain data consistency