Retrieve Navigation Property Dynamics WebAPI

I am trying to retrieve the properties of a child entity via the parent entity. General Ledger Account is the parent entity and I want the details for all General Ledger Action entities that are related to the General Ledger Account. General Ledger Action has a lookup field to General Ledger Account. I am using ajax and typescript to build the url's and retrieve the data.

According to Microsoft Documentation:

Unless you specify a smaller page size, a maximum of 5000 entities will be returned for each request. If there are more entities that match the query filter criteria, a @odata.nextLink property will be returned with the results. Use the value of the @odata.nextLink property with a new GET request to return the next page of data.

I am only returning 1 parent entity and 1 child entity so I shouldn't be hitting this barrier. However,

When I use the url:

https://crm.com/api/data/v8.0/ccseq_generalledgeraccounts(48513938-7d5a-e711-80e5-005056b33317)/ccseq_generalledgeraccount_ccseq_generalledgeraction_GeneralLedgerAccountID

I get the results:

{
  "@odata.context":"https://crm.com/api/data/v8.0/$metadata#ccseq_generalledgeractions","value":[
    {
      "@odata.etag":"W/\"676070510\"","_organizationid_value":"ff05a89b-16b0-44a6-879c-26866b3a9d9d","ccseq_servicecode":"111","modifiedon":"2017-06-26T14:41:49Z","ccseq_generalledgernumber":"44000","ccseq_dollar":25.0000,"statecode":0,"ccseq_percent":0.0000000000,"statuscode":1,"_createdby_value":"3b0731e3-52bd-e611-80df-005056b33317","ccseq_jobclasscode":"111","ccseq_companycode":"111","_ccseq_generalledgeraccountid_value":"48513938-7d5a-e711-80e5-005056b33317","ccseq_dollar_base":25.0000,"_modifiedby_value":"3b0731e3-52bd-e611-80df-005056b33317","versionnumber":676070510,"exchangerate":1.0000000000,"ccseq_generalledgeractionid":"3e17f993-7d5a-e711-80e5-005056b33317","ccseq_geographycode":"111","createdon":"2017-06-26T14:41:49Z","_transactioncurrencyid_value":"863aa006-cae5-dc11-92e8-001a6449bbe7","_modifiedonbehalfby_value":null,"_createdonbehalfby_value":null,"utcconversiontimezonecode":null,"timezoneruleversionnumber":null,"importsequencenumber":null,"ccseq_name":null,"overriddencreatedon":null
    }
  ]
}

When I use the url:

https://crm.com/api/data/v8.0/ccseq_generalledgeraccounts?$select=ccseq_code&$expand=ccseq_generalledgeraccount_ccseq_generalledgeraction_GeneralLedgerAccountID

I get the results:

{
  "@odata.context":"https://crm.com/api/data/v8.0/$metadata#ccseq_generalledgeraccounts(ccseq_code,ccseq_generalledgeraccount_ccseq_generalledgeraction_GeneralLedgerAccountID)","value":[
    {
      "@odata.etag":"W/\"676070286\"","ccseq_code":null,"ccseq_generalledgeraccountid":"48513938-7d5a-e711-80e5-005056b33317","ccseq_generalledgeraccount_ccseq_generalledgeraction_GeneralLedgerAccountID":[

      ],"ccseq_generalledgeraccount_ccseq_generalledgeraction_GeneralLedgerAccountID@odata.nextLink":"https://crminternal.cohencpa.com/COHEN/api/data/v8.0/ccseq_generalledgeraccounts(48513938-7d5a-e711-80e5-005056b33317)/ccseq_generalledgeraccount_ccseq_generalledgeraction_GeneralLedgerAccountID"
    }
  ]
}

My understanding is that these two url's should return the exact same data which would be a json object of the parent & child attributes. What is the difference in these queries? Why is my second query returning the odata.nextLink instead of the json representation of the entity?

The difference is that in your first request you're retrieving data from a single record and in the second example you're retrieving all the General Ledger Accounts.

As you can imagine at this point, the $expand works differently according if you're retrieving a single record or a collection (some time ago this was not even supported and you'd get an exception) to avoid performance issues. When retrieving collection, the WebAPI gives you the @odata.nextLink as you mentioned that you can use to retrieve the child records using a second GET.

You can see an example of this in the MSDN:

Retrieve related entities by expanding collection-valued navigation properties: If you expand on collection-valued navigation parameters to retrieve related entities for entity sets, an @odata.nextLink property will be returned for the related entities. You should use the value of the @odata.nextLink property with a new GET request to return the required data.