warn if link already posted to other communties

app/community/forms.py

@@ -124,7 +124,10 @@ class CreateDiscussionForm(CreatePostForm):
 
 class CreateLinkForm(CreatePostForm):
     link_url = StringField(_l('URL'), validators=[DataRequired(), Regexp(r'^https?://', message='Submitted links need to start with "http://"" or "https://"')],
-                           render_kw={'placeholder': 'https://...'})
+                           render_kw={'placeholder': 'https://...',
+                                      'hx-get': '/community/check_url_already_posted',
+                                      'hx-params': '*',
+                                      'hx-target': '#urlUsed'})
 
     def validate(self, extra_validators=None) -> bool:
         domain = domain_from_url(self.link_url.data, create=False)

app/community/routes.py

@@ -2,7 +2,9 @@ from collections import namedtuple
 from io import BytesIO
 from random import randint
 
-from flask import redirect, url_for, flash, request, make_response, session, Markup, current_app, abort, g, json
+import flask
+from flask import redirect, url_for, flash, request, make_response, session, Markup, current_app, abort, g, json, \
+    jsonify
 from flask_login import current_user, login_required
 from flask_babel import _
 from slugify import slugify
@@ -36,7 +38,7 @@ from app.utils import get_setting, render_template, allowlist_html, markdown_to_
     joined_communities, moderating_communities, blocked_domains, mimetype_from_url, blocked_instances, \
     community_moderators, communities_banned_from, show_ban_message, recently_upvoted_posts, recently_downvoted_posts, \
     blocked_users, post_ranking, languages_for_form, english_language_id, menu_topics, add_to_modlog, \
-    blocked_communities
+    blocked_communities, remove_tracking_from_link
 from feedgen.feed import FeedGenerator
 from datetime import timezone, timedelta
 from copy import copy
@@ -1804,6 +1806,17 @@ def lookup(community, domain):
                 return redirect('/')
 
 
+@bp.route('/check_url_already_posted')
+def check_url_already_posted():
+    url = request.args.get('link_url')
+    if url:
+        url = remove_tracking_from_link(url.strip())
+        communities = Community.query.filter_by(banned=False).join(Post).filter(Post.url == url).all()
+        return flask.render_template('community/check_url_posted.html', communities=communities)
+    else:
+        abort(404)
+
+
 def upvote_own_post(post):
         post.score = 1
         post.up_votes = 1

app/templates/community/check_url_posted.html

@@ -0,0 +1,7 @@
+{% if communities -%}
+    <p>{{ _('This link has already been posted to these communities:') }}<br>
+    {% for community in communities -%}
+        {{ community.display_name() }}<br>
+    {% endfor -%}
+    </p>
+{% endif %}

app/templates/post/post_edit.html

@@ -15,6 +15,7 @@
             {{ render_field(form.title) }}
             {% if post_type == POST_TYPE_LINK %}
                 {{ render_field(form.link_url) }}
+                <div id="urlUsed"></div>
             {% elif post_type == POST_TYPE_IMAGE %}
                 {{ render_field(form.image_file) }}
                 {{ render_field(form.image_alt_text) }}