Posting process

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Posting process
Process of posting and editing messages in Misago is fully modular and extensible.
Whenever there is a need to allow user to post or edit existing message, special pipeline object is initialized and passed HttpRequest object istance as well as instances of current user, forum, thread, post and post you are replying to. This pipeline then instiates classes defined in MISAGO_POSTING_MIDDLEWARES setting.
This pipeline then asks its middlewares if they have any forms they want to display on posting page, or if they have any actions to perform before, on, or after save to database.
Pipeline itself performs no changes in models or database state, delegating all tasks to its middlewares.
Pipeline is also isolated in database transaction with locks on database rows taking part in posting process. This means that its protected from eventual race conditions and in cause of eventual errors or interruptions, can be rolled back without any downsides.
Note
Attachments and any other extra data stored outside of Database that was changed before rollback was called is excluded from this.
Writing custom middlewares
Middlewares are classes extending PostingMiddleware class defined in misago.threads.posting package.
During pipeline's initialization each class defined in MISAGO_POSTING_MIDDLEWARES setting is initialized and passed current context, which is then stored on its object attributes:
mode - Integer used to identify type of action. You can compare it with START, REPLY and EDIT constants defined in misago.threads.posting package.
request - HttpRequest using pipeline.
user - Authenticated user writing message.
category - Category in which message will be posted.
thread - Depending on mode and phase in posting process, this may be thread loaded from database, or empty model that is yet to be saved in database.
post - Depending on mode and phase in posting process, this may be post loaded from database, or empty model that is yet to be saved in database.
quote - If mode is REPLY, this may be None or instance of message to which reply is written by user.
datetime - Instance of datetime returned by django.utils.timezone.now call. You can use it to avoid subtle differences betwen dates stored on different models generated by new calls to timezone.now.
parsing_result - dictionary containing result of parsing message entered by user.
After middleware object has been created, its use_this_middleware method is called with no arguments to let middleware make decision if it wants to participate in process and return True or False accordingly. If you don't define this method yourself, default definition always returns True.
In addition to use_this_middleware method mentioned ealier, each middleware may define following methods to participate in different phases of posting process:
make_form()
Allows middlewares to display and bind to data their own forms. This function is expected to return None (default behaviour) or Form class instance.
Forms returned by middlewares need to have two special attributes:
legend - Name of form (eg. "Poll").
template - String with path to template that will be used to render this form.
In addition to this form has to declare itself as either main or supporting form via defining is_main or is_supporting attribute with value being True.
If form requires inclusion of JavaScript to work, js_template may be defined with path to template to be rendered before document's </body>.
pre_save(form)
This method is called with either middleware's form instance or None before any state was saved to database.
save(form)
This method is called with either middleware's form instance or None to save state to database.
post_save(form)
This method is called with either middleware's form instance or None to perform additional actions like sending notifications, etc. ect..
Reducing number of database saves
Misago provides custom SaveChangesMiddleware middleware that allows other middlewares to combine database saves. This middleware is called at the end of save and post_save phrases, meaning its possible for other middlewares to delegate saves to it during any time of posting process.
This middleware works by defining two extra attributes on User, Forum, Thread and Post models: update_all and update_fields.
If middleware made changes to many fields on model, changing it's update_all attribute to True will make SaveChangesMiddleware call model's save method with no arguments at the end of either save or post_save phase.
However if middleware changed only few fields, it may be better to append their names to model's update_fields attribute instead.
When different middlewares add custom fields to update_fields and set update_all flag at same time, Misago will perform one save() call updating whole model.
Note
Do not make assumptions or "piggyback" on other middlewares save orders. Introspecting either update_all or update_fields is bad practice. If your middlware wants its changes saved to database, it should add changed fields names to update_fields list even if it already contains them, or set update_all to True.
Interrupting posting process from middleware
Middlewares can always interrupt (and rollback) posting process during interrupt_posting phrase by raising misago.threads.posting.PostingInterrupt exception with error message as its only argument.
All PostingInterrupts raised outside that phase will be escalated to ValueError that will result in 500 error response from Misago. However as this will happen inside database transaction, there is chance that no data loss has occured in the process.
Performing custom validation
Misago defines custom validation framework for posts very much alike one used for validating new registrations.
This framework works by passing user post, title, its parsing result as well as posting middleware's context trough list of callables imported from paths specified in the MISAGO_POST_VALIDATORS settings.
Each serializer is expected to be callable accepting two arguments:
context dict with context that was passed to the posting middleware.
data dict with cleaned data: post containing raw input entered by user and parsing_result, an dict defining parsed_text key containing parsed message, mentions with list of mentions, images list of urls to images and two lists: outgoing_links and internal_links. In case of user posting new thread, this dict will also contain title key containing cleaned title.
Your validator should raise from rest_framework.serializers.ValidationError on error. If validation passes it may return nothing, or updated data dict, which allows validators to perform last-minute cleanups on user input.