How many rows does the preview load?

Just the first rows of your source, enough to read the headers and show a sample value. It is fast and does not pull your whole catalog.

The dropdowns are empty. Why?

You have not loaded a preview yet, or the preview could not read your source. Click "Load preview". If it errors, check the source URL on the Source tab.

Does the sample include my transforms?

The Fields-tab sample shows the raw mapped value. To see transforms and rules applied, use the live preview on the Output tab, and "Preview & validate" on the Publish tab for the final channel file.

Why does it remember an old sample?

So the mapping screen is not empty when you come back. It shows the last fetched sample with a timestamp. Click "Refresh preview" for current data.

Preview fields and the live sample

See real data from your source as you map. Load a preview, read your actual columns, and check sample values before you save.

Why preview first

Mapping is much easier when you can see your real data. The preview fetches the first rows of your source so MartechFlow knows your exact column names and can show a sample value next to each field.

Without a preview the dropdowns would be guessing. With one, every field shows the actual column choices from your file and the first row's value for whatever you pick.

Load and refresh the preview

On the Fields tab, click "Load preview". MartechFlow reads the first rows of your source URL (or your connected source) and lists your columns. If your source is temporarily blocked by a firewall, MartechFlow will try its fallback fetch so you can still preview and map.

Your most recent sample is remembered. When you return to the feed, the dropdowns and samples appear right away with a "Last fetched" time and whether the sample came from a preview or from your last sync. Click "Refresh preview" to pull fresh rows whenever you want.

1Open the feed and go to the Fields tab.
2Click "Load preview" (it becomes "Refresh preview" afterward).
3Watch each field's Sample column populate as you choose source columns.

Read the live sample as you map

Next to every field is a Sample cell. When you pick a source column, the sample shows that column's value from the first previewed row. This is the fastest way to confirm you mapped the right column: if the Title sample shows a price, you picked the wrong one.

If a field shows "no sample", it means you have not chosen a source column for it yet. That is fine for fields you intend to leave blank.

Preview the finished output too

The Fields-tab sample shows raw mapped values. To see your transforms and output rules applied, the Output tab carries its own live preview that runs your column rules on real product rows and shows the resulting value under each column.

When you are ready to publish, the Publish tab's "Preview & validate" goes one step further: it builds a sample of the actual channel file (XML or CSV), counts errors and warnings, and flags any required field that comes out empty, so you catch problems before a channel does.

If the preview fails

A failed preview almost always means the source itself could not be read: a wrong URL, a 403 or 404, a timeout, an address that is blocked for safety, or a file MartechFlow could not parse. The error message tells you which.

Fix the source URL on the Source tab (or the upload), then load the preview again. A blocked feed shows extra guidance on how to unblock it.