Help with List.Generate() in making API calls

[ChadC]

Hi All, 

I'm building a custom connector to utilize an API. I'm attempting to handle pagination, but am having a hard time. Here's what I'm trying to do:

1. An call is made to the API.
2. A response is returned.
3. The connector needs to check if the response contains a header called "Continuation."
4. If true, another call needs to be made with the previous "Continuation" header included.
5. If false, return the results.

I'm trying to use List.Generate because it appears to be my best option for a Do...While/Until loop. Here's what I have:

Main = (Scope) =>
    let
        HasContToken = (responseHeaders) as logical => if (Record.HasFields(responseHeaders, {"Continuation"})) then 
                    true
                 else
                    false,
        GetPage = (Scope, ContinuationToken) =>
            let
                url = api_source_uri & Scope,
                RawData = Web.Contents(url, [
                    Headers = [
                        #"Continuation" = ContinuationToken
                   ]])
            in
                RawData,
        Pages = List.Generate(() => Web.Contents(api_source_uri & Scope),
                each HasContToken(Value.Metadata(_)[Headers]),
                each GetPage(Scope, Value.Metadata(_)[Headers][Continuation])),
        Pages_JSON = Json.Document(Pages),
        Table = Table.FromList(Pages_JSON, Splitter.SplitByNothing())
    in
        Table;

 I receive this error when trying to run this code: "The parameter is expected to be of type Text.Type or Binary.Type."

I don't know which parameter this is referring to, but I assume it is related to how I've set up List.Generate. Any help is greatly appreciated.

Thank you!

[v-xicai]

Hi @ChadC ,

When combining files, be sure to choose "Skip files with errors." see more: https://www.sqlchick.com/entries/2018/5/6/querying-data-in-azure-data-lake-store-with-power-bi.

There may be some nulls in your dataset which generated the error when the parsing was attempted, replace the null values with an empty structure "<div></div>" which was then parsed . See The parameter is expected to be of type Text.Type or Binary.Type.

If you still have this issue for Power BI, you'd better create a support ticket in Power BI Support , Scroll down and click "CREATE SUPPORT TICKET", to get further help.

Best Regards,

Amy

If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.

[ChadC · Accepted solution]

Hi @v-xicai ,

Thank you for your response. The issue was indeed related to null values being returned in subsequent calls. However, this was because the API I'm using required an "accept" header to be included, which I had overlooked. Below is my updated code:

let
        HasContToken = (responseHeaders) as logical => if (Record.HasFields(responseHeaders[Headers], {"Continuation"})) then 
                    true
                 else
                    false,
        GetPage = (Scope, ContinuationToken) =>
            let
                RawData = Web.Contents(api_source_uri, [
                    RelativePath = "/reporting/v1/" & Scope,
                    Query = [
                        limit = "1000"
                    ],
                    Headers = [
                        #"Continuation" = ContinuationToken,
                        accept = "application/json"
                   ]])
            in
                RawData,
        Pages = List.Generate(() => Web.Contents(api_source_uri, [
                    RelativePath = "/reporting/v1/" & Scope,
                    Headers = [
                        accept = "application/json"
                    ],
                    Query = [
                        limit = "1000"
                    ]]),
                each HasContToken(Value.Metadata(_)),
                each GetPage(Scope, Value.Metadata(_)[Headers][Continuation])),
        JSON = List.Transform(Pages, (_) => Json.Document(_)),
        List = List.Combine(JSON),
        Table = Table.FromList(List, Splitter.SplitByNothing(), null, null, ExtraValues.Error)
    in
        Table;