Understanding the elements of 'Source' in Power Query

Hi thomasfmeier ,

The error you encountered and its resolution likely relate to how Power Query handles schema changes in your data source, particularly the addition of new columns. Let's break down the elements and address your questions:

Here's an example of how to make the Source step more flexible:

let
    Filepath = Excel.CurrentWorkbook(){[Name="Filepath"]}[Content]{0}[Column1],
    Source = Excel.Workbook(File.Contents(Filepath & "Hashtag Items.xlsx"), null, true),
    ColumnsToKeep = {"Column1", "Column2", "Column3"},  // Adjust based on your actual column names
    FilteredTable = Table.SelectColumns(Source, ColumnsToKeep)
in
    FilteredTable

This approach ensures that your query works even if additional columns are added to the file. 

I'll try to explain the method I used here in simple way with some examples

1. Error Due to Schema Mismatch: Power Query expects the source data's schema (e.g., column names and their order) to remain consistent. If columns are added, removed, or renamed in the source file, it may cause errors.

2. Resolution by Updating the File: Using a file with the additional columns resolved the issue because it matched the schema Power Query expected at that step.

Elements of the Query

• Source Step:

  = Excel.Workbook(File.Contents(Filepath & "Hashtag Items.xlsx"), null, true)

  • This reads the Excel file specified by Filepath and retrieves its contents as a workbook object.

• Filepath Step:

  Filepath = Excel.CurrentWorkbook(){[Name="Filepath"]}[Content]{0}[Column1]

  • Dynamically determines the file path, allowing flexibility in where you store the source files.

Why Is This Different?

1. Power Query determines the schema of the file at the time the query is created or refreshed. If the file's structure changes (e.g., new columns are added), it may not align with the query's expectations, causing errors.

2.  While the source system may not have changed, the version of the file used for the query might include updates (e.g., additional columns). This mismatch can cause errors.

Solutions to Handle Schema Changes

To make your query more robust against schema changes, you can update it as follows:

1. Dynamically Detect Columns:

   • Modify your query to handle dynamic column detection:

     = Table.SelectColumns(Source, List.FirstN(Table.ColumnNames(Source), X))

     Replace X with the number of columns you want to process, or adapt based on column names.

2. Ignore Extra Columns:

   • Use the Table.RemoveColumns function to remove unexpected columns:

     = Table.RemoveColumns(Source, List.Difference(Table.ColumnNames(Source), ExpectedColumns))

     Replace ExpectedColumns with a list of column names you want to retain.

3. Adjust Based on New Schema:

   • If you want Power Query to accommodate the new schema automatically, modify the steps referencing the columns. Use the "Column Index" instead of column names when possible.

Hi thomasfmeier 

Thanks for reaching out to the Microsoft forum community. Sorry for the delay in response, based on the concerns that you're facing issues when refreshing the report, especially related to schema changes in the source file. 

1. As long as the number of columns remains unchanged and only the data within those columns is updated (as expected during a monthly refresh), your query should function without any issues. Problems occur when the structure of the file is altered. 
2. If column names are changed, Power Query will attempt to match the query to the old names and will generate an error if the columns no longer exist with their expected names. To address this, you can use a more dynamic approach to accommodate renamed columns. 

• If the file’s structure changes often, it’s worth updating the query to dynamically detect columns to handle future schema changes. 

If this post helps, please give us Kudos and consider Accept it as a solution to help the other members find it more quickly. 

Thank you.