Make row available to `render_cell` plugin hook

[abdusco]

Original title: Generating URL for a row inside render_cell hook

Hey,
I am using Datasette to view a database that contains video metadata. It has BLOB columns that contain video thumbnails in JPG format (around 100-500KB per row).

I've registered an output formatter that extends datasette.blob_renderer.render_blob function and serves the column with image/jpeg content type.

from datasette.blob_renderer import render_blob

async def render_jpg(datasette, database, rows, columns, request, table, view_name):
    response = await render_blob(datasette, database, rows, columns, request, table, view_name)
    response.content_type = "image/jpeg"
    response.headers["Content-Disposition"] = f'inline; filename="image.jpg"'
    return response


@hookimpl
def register_output_renderer():
    return {
        "extension": "jpg",
        "render": render_jpg,
        "can_render": lambda: True,
    }

This works well. I can visit http://localhost:8001/mydb/videos/1.jpg?_blob_column=thumbnail and view the image.

I want to display the image directly with an <img> tag (lazy-loaded of course). So, I need a URL, because embedding base64 would increase the page size too much (each image > 100KB).

Datasette generates a link with .blob extension for blob columns. It does this by calling datasette.urls.row_blob

datasette/datasette/views/table.py

Lines 169 to 179 in 7a2ed9f

	display_value = jinja2.Markup(
	'<a class="blob-download" href="{}">&lt;Binary:&nbsp;{}&nbsp;byte{}&gt;</a>'.format(
	self.ds.urls.row_blob(
	database,
	table,
	path_from_row_pks(row, pks, not pks),
	column,
	),
	len(value),
	"" if len(value) == 1 else "s",
	)

But I have no way of getting the row inside the render_cell hook.

@hookimpl
def render_cell(value, column, table, database, datasette):
    if isinstance(value, bytes) and imghdr.what(None, value):
        # generate url
        return '$renderedLink'

Any pointers?

[abdusco]

If I change the hookspec and add a row parameter, it works

datasette/datasette/hookspecs.py

Line 58 in 7a2ed9f

def render_cell(value, column, table, database, datasette):

def render_cell(value, column, row, table, database, datasette):

But to generate a URL, I need the primary keys, but I can't call pks = await db.primary_keys(table) inside a sync function. I can't call datasette.utils.detect_primary_keys either, because the db connection is not publicly exposed (AFAICT).

[abdusco]

If there's a simpler way to generate a URL for a specific row, I'm all ears

[abdusco]

I ended up using some JS as a workaround.

First, add a JS file in metadata.yaml:

extra_js_urls:
  - '/static/app.js'

then inside the script, find the blob download links and replace .blob extension in the url with .jpg and replace the links with <img/> elements.
You need to add an output formatter to serve BLOB columns as JPG. You can find the code in the first post.
Replacing .blob -> .jpg might not even be necessary, because browsers only care about the mime type, so you only need to serve the binary content with the right content-type header.. You need to replace the extension, otherwise the output renderer will not run.

window.addEventListener('DOMContentLoaded', () => {
    function renderBlobImages() {
        document.querySelectorAll('a[href*=".blob"]').forEach(el => {
            const img = document.createElement('img');
            img.className = 'blob-image';
            img.loading = 'lazy';
            img.src = el.href.replace('.blob', '.jpg');
            el.parentElement.replaceChild(img, el);
        });
    }

    renderBlobImages();
});

while this does the job, I'd prefer handling this in Python where it belongs.

[simonw]

Adding row to this plugin hook is a smart idea.

[simonw]

Updated documentation: https://docs.datasette.io/en/latest/plugin_hooks.html#render-cell-row-value-column-table-database-datasette