I'm working on a Microsoft Access homework project. My question is: how to to make a data input form like:
"a. Construct a form that will collect sales data as input, and post it to the Sales table. Try to determine how to lock the existing records in the involved tables, so that the data in those records cannot be modified in the Form. In other words, the Form is to be used only for entering new sales and reviewing existing/previous sales, but not for altering existing records."
I've searched for tutorials on how to make forms but they are all extremely basic and don't provide enough details to help me reach the requested result. Is there a template or a design trace close to the requested form?
Set property AllowEdits of the form to False.
But this is bad design. When you have saved a new record and notice a typo or something, you can't edit it.
Related
The title may not be as accurate, if someone finds a better one and can update it, please do so :)
I have a small CMS to edit users.
I'm using Zend Framework 2 + Doctrine 2.
I have a fieldset + form to add a user and (possibly) the same one to update them.
The User entity has the following fields:id, username, password, email.
The fieldset has two validators that check if the username and the email already exist.
Since I'm using the same to update the users, when I change for example the username of the user and keep the email the same, it throws an error that the "email exists" (which is normal due to the validator) and the same when I change the username and keep the email etc.
What I want is to avoid that behavior and make it so it checks them only when they are really changed/updated.
I thought of some ways, but I'm not sure what the "best" approach would be to this.
Hardcode the whole thing, by checking if the fields change and then do the validation (which makes the whole fieldset pretty much useless)
Make a function in the User entity that accepts an array with the new values, then compares them to the old ones and passes the changed ones to a "validation" function that returns the errors (which is mostly like the previous way, but I guess a bit more structured)
Write a validator and attach it to a new form which will query the db to check if the email/username exists and it's not already in use by the particular id, but I'm not quite sure on how to write it since I can't figure out how to pass the id and the field to the validator
I guess the 3rd one would be the best since it does 2 jobs at one time, by checking if the field changed and is not already in use by another user.
What do you suggest? How do you deal with that kind of scenario?
I can post any code that is needed, but I think this is more of a structural problem and that the code I used is too common and easy to figure out.
I'm building an InfoPath form connected to a SharePoint List.
After the user has filled out a couple of fields (which can uniquely identify a record), I re-query the main data connection to see if the item already exists. This works fine and loads the remaining details of the existing record into the form.
However, if the record does not exist, all the fields on the form become disabled... how can I reset the main data connection back to its initial state to allow a new item to be submitted?
You will need to set a condition rule. So for example on the last field the do an insert on do your query, then in your rules (you may need multiple depending on how many fields) , saying if field is (whatever) then do (whatever). The disable features are set in the formatting section of your rule.
Baseline info:
I'm using an external OAuth provider for login. If the user logs into the external OAuth, they are OK to enter my system. However this user may not yet exist in my system. It's not really a technology issue, but I'm using JOliver EventStore for what it's worth.
Logic:
I'm not given a guid for new users. I just have an email address.
I check my read model before sending a command, if the user email
exists, I issue a Login command with the ID, if not I issue a
CreateUser command with a generated ID. My issue is in the case of a new user.
A save occurs in the event store with the new ID.
Issue:
Assume two create commands are somehow issued before the read model is updated due to browser refresh or some other anomaly that occurs before consistency with the read model is achieved. That's OK that's not my problem.
What Happens:
Because the new ID is a Guid comb, there's no chance the event store will know that these two CreateUser commands represent the same user. By the time they get to the read model, the read model will know (because they have the same email) and can merge the two records or take some other compensating action. But now my read model is out of sync with the event store which still thinks these are two separate entities.
Perhaps it doesn't matter because:
Replaying the events will have the same effect on the read model
so that should be OK.
Because both commands are duplicate "Create" commands, they should contain identical information, so it's not like I'm losing anything in the event store.
Can anybody illuminate how they handled similar issues? If some compensating action needs to occur does the read model service issue some kind of compensation command when it realizes it's got a duplicate entry? Is there a simpler methodology I'm not considering?
You're very close to what I'd consider a proper possible solution. The scenario, if I may summarize, is somewhat like this:
Perform the OAuth-entication.
Using the read model decide between a recurring visitor and a new visitor, based on the email address.
In case of a new visitor, send a RegisterNewVisitor command message that gets handled and stored in the eventstore.
Assume there is some concurrency going on that, for the same email address, causes two RegisterNewVisitor messages, each containing what the system thinks is the key associated with the email address. These keys (guids) are different.
Detect this duplicate key issue in the read model and merge both read model records into one record.
Now instead of merging the records in the read model, why not send a ResolveDuplicateVisitorEmailAddress { Key1, Key2 } towards your domain model, leaving it up to the domain model (the codified form of the business decision to be taken) to resolve this issue. You could even have a dedicated read model to deal with these kind of issues, the other read model will just get a kind of DuplicateVisitorEmailAddressResolved event, and project it into the proper records.
Word of warning: You've asked a technical question and I gave you a technical, possible solution. In general, I would not apply this technique unless I had some business indicator that this is worth investing in (what's the frequency of a user logging in concurrently for the first time - maybe solving it this way is just a way of ignoring the root cause (flakey OAuth, no register new visitor process in place, etc)). There are other technical solutions to this problem but I wanted to give you the one closest to what you already have in place. They range from registering new visitors sequentially to keeping an in-memory projection of the visitors not yet in the read model.
I'm writing a basic event registration web application and I'm wondering whether MongoDB would be a good choice for the datastore and if so, how to model my domain. The app will be very small, so performance and scalability is not a concern, however when I started to think out the model in RDBMS third-normal terms it sounded quite complicated for what it is and from the bits and pieces I'm picking up about Mongo, sounded like a typical use-case. Is it?
The Application
The app allows creation of events, and for attendees to sign up to those events, giving their name, date of birth, etc. Easy, two tables with n:n join. The tricky part is that the organisers wish to be able to ask attendees of certain events for information particular to that event, for example on one event there might be a question about their accomodation preference. I narrowed it down to two types of question: those that require to select from certain options (will be an HTML select list) and questions which allow free-text answers. By the way it's a Rails app in case that matters.
Traditional RDBMS
In an RDBMS I would need perhaps a table for Constrained Question (where answers are from a list), a table for Answer Options, a table for Free Text Question and Free Text Answers; and to appropriately link this all up to the event and the atendee via a Signup. If you think about it the links between the tables are rather complicated!
Mongo
Would this be simpler to model in Mongo? I thought that perhaps besides the Attendee and Event collections, there could be a Question collection which has its allowed answers embedded, if there are no answers then it's free text. A Signup collection that relates an Attendee to an Event and references the id of the relevant Question, and embeds the text of the answer? If the text of an answer option ever changes it might get complicated... but I guess that's the tradeoff of Mongo.
Is this a good use case for Mongo on should I stick with Postgres? Can you suggest a (or improve my) schema?
Mongodb is a awesome tool for this job. You can pretty much utilize the embedded collection here to maximize the performance.
Your current schema is perfectly fine. By tweaking this a little bit with embedded collections , it will be a blast.
For instance, instead of keeping the Question collection seperately, you can have this inside Attendee. This will let you store all relevant info about the attendee in single place.
- Attendee
- Info
- Event_id
- Questions {
-Question id
- Answers [ {
- answer id 1
- or answer text
},{
- answer id 2
- or answer text
}],
}
Also you can cache the frequently used data about the attendees inside the Event collection.
This will be immensely useful for quickly displaying home page data.
For example, you may need to display the users who are attending the event and their count in the event home page. To do that you need to first query the event and query the Attendees.
But i suggest you to do store the attendee_id/name inside the Event as an array, which looks
Event :
- Info
- attendees {
attendee_id : 'xx'
name : 'Fletch'
}
So you can populate the event home page with a single db call to Event collection. Because you can get a minimum info to be displayed about the user and total user count for an event will be retrieved here itself. You can query the attendee when you need to display more info about the user like his question/answers.
Hope this helps
The repository in the CommonDomain only exposes the "GetById()". So what to do if my Handler needs a list of Customers for example?
On face value of your question, if you needed to perform operations on multiple aggregates, you would just provide the ID's of each aggregate in your command (which the client would obtain from the query side), then you get each aggregate from the repository.
However, looking at one of your comments in response to another answer I see what you are actually referring to is set based validation.
This very question has raised quite a lot debate about how to do this, and Greg Young has written an blog post on it.
The classic question is 'how do I check that the username hasn't already been used when processing my 'CreateUserCommand'. I believe the suggested approach is to assume that the client has already done this check by asking the query side before issuing the command. When the user aggregate is created the UserCreatedEvent will be raised and handled by the query side. Here, the insert query will fail (either because of a check or unique constraint in the DB), and a compensating command would be issued, which would delete the newly created aggregate and perhaps email the user telling them the username is already taken.
The main point is, you assume that the client has done the check. I know this is approach is difficult to grasp at first - but it's the nature of eventual consistency.
Also you might want to read this other question which is similar, and contains some wise words from Udi Dahan.
In the classic event sourcing model, queries like get all customers would be carried out by a separate query handler which listens to all events in the domain and builds a query model to satisfy the relevant questions.
If you need to query customers by last name, for instance, you could listen to all customer created and customer name change events and just update one table of last-name to customer-id pairs. You could hold other information relevant to the UI that is showing the data, or you could simply hold IDs and go to the repository for the relevant customers in order to work further with them.
You don't need list of customers in your handler. Each aggregate MUST be processed in its own transaction. If you want to show this list to user - just build appropriate view.
Your command needs to contain the id of the aggregate root it should operate on.
This id will be looked up by the client sending the command using a view in your readmodel. This view will be populated with data from the events that your AR emits.