So I've started learning access due to necessity, as the person who was in charge of it passed way and someone had to keep it going.
I noticed a very bad (at least IMO) behavior in all databases he created: Every single form was bound directly to a table or saved query. This way, if the user opened a form, he had to complete all the steps he was supposed to do, because if he closed the form mid process (or the computer froze, or anything of the sort), the actual data would be compromised as it would be half complete. This often times broke everything in the process chain, rendering sub-sequential steps impossible to be performed and forced me to correct data manually directly in the tables.
As I've start upgrading his stuff and developing my own, I've been trying to learn ways to allow the data to be edited in the form only, making it possible to cancel the process anytime or save the changes all at once in the end.
If the editions were simple, I discovered that I could create a recordset, copy relevant data to unbound fields in the form and, in the end, if the user chose to, copy the data from the form fields back to the recordset.
Other times more complex solutions were required, as I would need to edit several pieces of data at once in continuous forms, "save" them, run more code, maybe add fields to hold the information originated from that processing and so on. I then learned about using temporary tables, but did not like it, since it tended to bloat the db. I even went on to creating temporary databases during code execution that would host the temporary tables and be destroyed in the end, but that added too much unnecessary complexity.
Nowadays I'm using disconnected ADO recordsets to hold the temporary data and fields. It works but has its limitations.
So I'm wondering, what is the best way you - much more experienced than me - guys use to approach this kind of scenario? Is using in memory ADO recordsets really the best way around?
I think you are mixing two things that a form does that have completely different requirements. Editing existing records (and bound forms are great for that) and creating new records (where using a straight bound form can result in creating incomplete records). The way to approach it depends on many things but mainly to how much data is necessary for a new record to be considered "complete".
I usually do one of the following things:
Create an unbound popup modal form for adding new records where only the necessary fields are present. Once complete it loads the new record to the main form for further editing.
Use the above method except the form is not a popup one but a set of unbound fields in the footer or header of the main form.
Let the user create new records but bind validation on the OnClose (and/or other appropriate to your situation) event of the form that deletes the half-filled record if it does not validate.
Let users create new records in the bound form but have a 'cleanup' routine called either on a schedule or based on user actions.
Ultimately if your business process requires the majority of fields to be manually added/edited every time a new record is added or edited, you are better off using an unbound form.
This way, if the user opened a form, he had to complete all the steps
he was supposed to do, because if he closed the form mid process (or
the computer froze, or anything of the sort), the actual data would be
compromised as it would be half complete
No, if the computer freezes, then no data is saved to the table. This is the same if you used a disconnected reocrdset and a un-bound form.
If you use the before update event in the form that has some verification code and does a simple cancel = true, then the forms data is not saved nor is the table updated. Again, if you used a dis-connected record set and the user closes the form, you have to test the data – and again you can either choose to write out the data or not – this effect is ZERO difference from using a bound form to a table or a disconnected form.
If the editions were simple, I discovered that I could create a
recordset, copy relevant data to unbound fields in the form and, in
the end, if the user chose to, copy the data from the form fields back
to the recordset.
No you don’t need to do the above. The above achieves nothing and only racks up additional development hours and increases cost of the application. In near all cases in-bound forms increase development costs over that of a simple form bound to a table. So the original developer had the correct idea. You can control the update of the underlying table in near all cases to achieve the required verification. Forms only save and write the data out if the developer allows as such.
So Access forms when bound no more or less write incomplete data out to a table if you place verification code in the forms before update event. A half-filled bound form, or a half filled un-bound form with dis-connected reocrdset BOTH will not write their data if the computer freezes.
And BOTH types of forms will not write out data to table until such time your verification code has completed.
Access is not designed for un-bound forms, and tools like vb.net, or even VB6 had a whole bunch of cool wizards and support for un-bound forms. In access, we don't have those wizards. And when you use UN-bound forms then you loose tons of form events. You in effect get the worst of both worlds, since you lose use of form events and have no wizards or support for un-bound. Even just the several delete record events we have are rather amazing.
You lose use of me.dirty, on-insert, me.newReocrd, forms after update events - the list of features you toss out and lose is huge. And if you want a button to write data to the table (such as a save button on the form), then just go:
If me.Dirty = True then
me.Dirty = False ' this forces your verification code to run
End if
There are FEW use cases in which in-bound forms will benefit you, but they will cost you rather much in terms of development times.
Related
I have an access project that is "linked" to a SQL database that now works like a charm. The last problem I solved was, making sure any Boolean fields be turned to bits with default of 0, and adding the TIMESTAMP in SQL due to the fact that ACCESS is not so much of a genius with record locking (so I was told) .
Now that I tried to connect direct to SQL server by using an ADODB.Recordset and setting the forms.recordset to the recordset, at the OnOpen event of the form, (this recordset runs a stored procedure in SQL, I get the data fine but get the error locking (write conflict) back.
This ADODB.Recordset cursorlocation is set to "adUseClient".
Obviously I no longer have the forms recordsource attached or assigned to the linked SQL table anymore.
Am I missing something? do I need to assign anything to the forms recordsource?
The Idea is trying to connect directly thru the use of stored procedures instead of linked tables.
thanks so much for any help.
The adding of timestamp is a VERY good idea. And do not confuse the term/name used timestamp to mean an actual date/time column. The correct term is "row version".
This issue has ZERO to do with locking. The REASON why you want this column added is because then Access will use that column to determine when the record is dirty, and more imporant figure out that the record been changed. If you omit this column, then access reverts to a column by column testing approach. Not only does this cause more network traffic, but worse for real type values, due to rounding, you can get the dredged this record has been changed by another user. But, it not been changed, and even columns with floating point values will cause access to error out with that changed record.
So, for all tables, and you even see the option included in the SSMA (the access to sql migration wizard that this option is available (and I believe it is a default).
So yes, it is HIGH but VERY high recommended that you include/add a rowversion column to all tables - this will help Access in a HUGE way.
And as noted, there is a long standing issue with bit fields that don't have a default setting. so, you don't want to allow bit fields to be added/created with a null value. So, ensure that there is a default value of 0 (you set this sql server side).
Ok, now that we have the above cleared up?
It not really all that clear as to why you want or need or are adopting a store procedure and code to load/fill up the form. You not see any better performance if you bind the form DIRECTLY to the linked table. Access will ONLY pull the reocrds you tell that form to load.
So, bind the form directly to the linked table. Then, you can launch/open the form say to once reocrd with this:
docmd.OpenForm "frmInvoices",,,"InvoiceNum = 123"
Now, you would of course change the above "123" to some variable or some way to prompt the user for what invoice to work on.
The invoice form will then load to the ONE record. So, even if the form bound (linked table) has 2 million rows? Only ONE record will come down the network pipe. So, all that extra work of a store procedure, creating a recordset and pulling it ? You will gain ZERO in terms of performance, but you are writing all kinds of code when it simply not required, and you not achieve any superior performance to the above one line of code that will automatic filter and ONLY pull down the record that meets the given criteria (in this example invoice number).
So:
Yes, all tables need a PK
Yes, all tables should have a rowversion (but it called a timestamp column - nothing to do with the actual time).
Yes, all bit fields need a default of 0 - don't allow null values.
And last but not least?
I don't see any gains in performance, or even any advantages of attempting to code your way though this by adopting store procedures and that of introducing reocrdset code when none is required, but worse will not gain you performance anyway.
As an amateur coder for some years I have generally used sub forms when dealing with linked tables to make the transfer of ID field from primary to sub nice and simple...
However in my latest project the main form is a continuous form with a list of delivery runs (Date, RunName, RunCompleted) etc... Linked to this primary table is a delivery list containing (SKU of product, Qty etc...). I use a simple Relationship between the two tables.
Now, On the main (RUNS) form at the end of each row is a button that opens the DELIVERIES form and displays all records with matching RUNID
This is fine for displaying pre-existing data but when I want to add new records I have been using the following code attached to the OnCurrent event:
Me.RunID = DLookup("[RunID]", "tbl_BCCRuns", "RunID = " & Forms![frm_BCC_Runs_list]![RunID])
I have also used:
Forms![frm_BCC_Deliveries].Controls![RunID] = Forms![tbl_BCCRuns].Controls![RunID]
(Note: above done from memory and exact code may be incorrect but that's not the problem at hand)
Now... Both these options give me what I need however...
I find that as I am working on the database, or if you open certain forms in the right order (a bug I need to identify and fix clearly) you can open the DELIVERIES form without the filter (to view all deliveries for arguments sake) and the top entry (usually the oldest record) suddenly adopts the RUNID of the selected record back in the main form.
Now, my question is this, and the answer may be a simple "no" and that's fine, I'll move on...
Is there a better way, a way I am not familiar with or just don't know about due to my inconsistent Access progress, to transfer ID's to a form without risking contamination from improper use? Or do I just have to bite the bullet and make sure that there is just no possible way for that to happen?
In effort to alleviate the issue, I have created a Display Only form for viewing the deliveries but there are still times when I need to access the live historical data to modify other fields without wanting to modify the RUNID.
Any pointers greatly appreciated...
Since you only want to pull the RunID if the form is on a new record row, do a check to verify this is a new record.
If Me.NewRecord Then
Me.RunID = DLookup("[RunID]", "tbl_BCCRuns", "RunID = " & Forms![frm_BCC_Runs_list]![RunID])
End If
Could also consider a technique to synchronize parent and child forms when both are subforms on a main form (the main form does not have to be bound) https://www.fmsinc.com/MicrosoftAccess/Forms/Synchronize/LinkedSubforms.asp
I have a problem with oracle apex forms.
The problem is that I want to add more than 1 record at the same time in 1 form. I have already read that the best way to do that is to use an csv file but then there is no tutorial to do that.
Oracle is a database, and combining files with databases is always tricky and not extensively supported for obvious reasons. Storing files and presenting them for download is one thing. Getting an Oracle database to open a file and reading and processing the contents is another. It sure it possible, but especially combining this with an Apex application I think you are going to run into a lot of challenges such as security restrictions.
However, stepping away from files does not necessarily mean stepping away from CSV. You could simply offer a large text input on your page in which a user can copy-paste a large CSV string. This can then be submitted and processed by the database. To do this you would probably need to create a process that gets fired after you submit the page. From this process you can parse the CSV data and insert multiple rows in a table. The same can be done for things like XML or JSON.
However, who is generating this CSV? Requiring a user to construct CSV is not very user-friendly. It can be complicated and error prone. If the CSV is generated by another application, isn't there a way to circumvent Apex and pass the CSV to the database directly?
If a single text-based data carrier is not required, which I doubt reading your descriptions, why not simply keep your form but allow the user to submit multiple forms? Would if be sufficient to insert one record per submit, and later using a batch to query these records and start formatting one at a time?
If you simply want the user to be able to enter multiple machines without having to submit the page for each machine, this is also possible, but you will have to leave some standard Apex functionality behind and implement some more custom javascript and PL/SQL functionality. Apex only allows a static amount of page items, which needs to be defined design time. So if you want to dynamically add fields such as text boxes and select lists to your page, you will have to resort to javascript. You could start by defining a region which renders one row of input fields at page load, and create a link under it saying 'add another row', which will render a new row of input fields under the existing one, and repeat this as many times as the user needs to.
That takes care of the UI. Now when the user has entered all the data he wants, we need to submit all this data and get it into the database. So yes, at this point we would probably have to get all this data from the input fields and turn it into one single string. This would all have to be done client side in your javascript code. You can then use the Apex page item API to assign this generated string to a single page item, using the $x(...) or $v(...) functions. Then submit the page, at which point the page processes will be fired. You then define a page process which will parse the data in your page item, and use that data to insert multiple rows in the database.
We've got a set of forms in our web application that is managed by multiple staff members. The forms are common for all staff members. Right now, we've implemented a locking mechanism. But the issue is that there's no reliable way of knowing when a user has logged out of the system, so the form needs to be unlocked. I was wondering if there was a better way to manage concurrent users editing the same data.
You can use optimistic concurrency which is how the .Net data libraries are designed. Effectively you assume that usually no one will edit a row concurrently. When it occurs, you can either throw away the changes made, or try and create some nicer retry logic when you have two users edit the same row.
If you keep a copy of what was in the row when you started editing it and then write your update as:
Update Table set column = changedvalue
where column1 = column1prev
AND column2 = column2prev...
If this updates zero rows, then you know that the row changed during the edit and you can then deal with it, or simply throw an error and tell the user to try again.
You could also create some retry logic? Re-read the row from the database and check whether the change made by your user and the change made in the database are able to be safely combined, then do so automatically. Or you could present a choice to the user as to whether they still wish to make their change based on the values now in the database.
Do something similar to what is done in many version control systems. Allow anyone to edit the data. When the user submits the form, the database is checked for changes. If the record has not been changed prior to this submission, allow it as usual. If both changes are the same, ignore the incoming (now redundant) change.
If the second change is different from the first, the record is now in conflict. The user is presented with a new form, which indicates which fields were changed by the conflicting update. It is then the user's responsibility to resolve the conflict (by updating both sets of changes), or to allow the existing update to stand.
As Spence suggested, what you need is optimistic concurrency. A standard website that does no accounting for whether the data has changed uses what I call "last write wins". Simply put, whichever connection saves to the database last, that version of the data is the one that sticks. In optimistic concurrency, you use a "first write wins" logic such that if two connections try to save the same row at the same time, the first one that commits wins and the second is rejected.
There are two pieces to this mechanism:
The rules by which you fail the second commit
How the system or the user handles the rejected commit.
Determining whether to reject the commit
Two approaches:
Comparison column that changes each time a commit happens
Compare the data with its committed version in the database.
The first one entails using something like SQL Server's rowversion data type which is guaranteed to change each time the row changes. The upside is that it makes it simple to roll your own logic to determine if something has changed. When you get the data, you pull the rowversion column's value and when you commit, you compare that value with what is currently in the database. If they are different, the data has changed since you last retrieved it and you should reject the commit otherwise proceed to save the data.
The second one entails comparing the columns you pulled with their existing committed values in the database. As Spence suggested, if you attempt the update and no rows were updated, then clearly one of the criteria failed. This logic can get tricky when some of the values are null. Many object relational mappers and even .NET's DataTable and DataAdapter technology can help you handle this.
Handling the rejected commit
If you do not leave it up to the user, then the form would throw some message stating that the data has changed since they last edited and you would simply re-retrieve the data overwriting their changes. As you can imagine, users aren't particularly fond of this solution especially in a high volume system where it might happen frequently.
A more sophisticated (and also more complicated) approach is to show the user what has changed allow them to choose which items to try to re-commit, Behind the scenes you would retrieve the data again, overwrite the values picked by the user with their entries and try to commit again. In high volume system, this will still be problematic because by the time the user has tried to re-commit, the data may have changed yet again.
The checkout concept is effectively pessimistic concurrency where users "lock" rows. As you have discovered, it is difficult to implement in a stateless environment. Users are notorious for simply closing their browser while they have something checked out or using the Back button to return a set that was checked out and try to recommit it. IMO, it is more trouble than it is worth to try go this route in a web-based solution. Assuming you write the user name that last changed a given row, with optimistic concurrency, you can inform the user whose changes are rejected who saved the data before them.
I have seen this done two ways. The first is to have a "checked out" column in your database table associated with that data. Your service would have to look for this flag to see if it is being edited. You can have this expire after a time threshold is met (with a trigger) if the user doesn't commit changes. The second way is having a dedicated "checked out" table that stores id's and object names (probably the table name). It would work the same way and you would have less lookup time, theoretically. I see concurrency issues using the second method, however.
Why do you need to look for session timeout? Just synchronize access to your data (forms or whatever) and that's it.
UPDATE: If you mean you have "long transactions" where form is locked as soon as user opens editor (or whatever) and remains locked until user commits changes, then:
either use optimistic locking, implement it by versioning of forms data table
optimistic locking can cause loss of work, if user have been away for a long time, then tried to commit his changes and discovered that someone else already updated a form. In this case you may want to implement explicit "locking" of form, where user "locks" form as soon as he starts work on it. Other user will notice that form is "locked" and either communicate with lock owner to resolve issue, or he can "relock" form for himself, loosing all updates of first user in process.
We put in a very simple optimistic locking scheme that works like this:
every table has a last_update_date
field in it
when the form is created
the last_update_date for the record
is stored in a hidden input field
when the form is POSTED the server
checks the last_update_date in the
database against the date in the
hidden input field.
If they match,
then no one else has changed the
record since the form was created so
the system updates the data.
If they don't match, then someone else has
changed the record since the form was
created. The system sends the user back to the form edit page and tells the user that someone else edited the record and they must reapply their changes.
It is very simple and works well enough.
You can use "timestamp" column on your table. Refer: What is the mysterious 'timestamp' datatype in Sybase?
I understand that you want to avoid overwriting existing data with consecutively updates.
If so, when the user opens a screen you have to get last "timestamp" column to the client.
After changing data just before update, you should check the "timestamp" columns(yours and db) to make sure if anyone has changed tha data while he is editing.
If its changed you will alert an error and he has to startover. If it is not, update the data. Timestamp columns updated automatically.
The simplest method is to format your update statement to include the datetime when the record was last updated. For example:
UPDATE my_table SET my_column = new_val WHERE last_updated = <datetime when record was pulled from the db>
This way the update only succeeds if no one else has changed the record since the last read.
You can message to the user on conflict by checking if the update suceeded via a SELECT after the UPDATE.
I keep seeing questions floating through that make reference to a column in a database table named something like DateLastUpdated. I don't get it.
The only companion field I've ever seen is LastUpdateUserId or such. There's never an indicator about why the update took place; or even what the update was.
On top of that, this field is sometimes written from within a trigger, where even less context is available.
It certainly doesn't even come close to being an audit trail; so that can't be the justification. And if there is and audit trail somewhere in a log or whatever, this field would be redundant.
What am I missing? Why is this pattern so popular?
Such a field can be used to detect whether there are conflicting edits made by different processes. When you retrieve a record from the database, you get the previous DateLastUpdated field. After making changes to other fields, you submit the record back to the database layer. The database layer checks that the DateLastUpdated you submit matches the one still in the database. If it matches, then the update is performed (and DateLastUpdated is updated to the current time). However, if it does not match, then some other process has changed the record in the meantime and the current update can be aborted.
It depends on the exact circumstance, but a timestamp like that can be very useful for autogenerated data - you can figure out if something needs to be recalculated if a depedency has changed later on (this is how build systems calculate which files need to be recompiled).
Also, many websites will have data marking "Last changed" on a page, particularly news sites that may edit content. The exact reason isn't necessary (and there likely exist backups in case an audit trail is really necessary), but this data needs to be visible to the end user.
These sorts of things are typically used for business applications where user action is required to initiate the update. Typically, there will be some kind of business app (eg a CRM desktop application) and for most updates there tends to be only one way of making the update.
If you're looking at address data, that was done through the "Maintain Address" screen, etc.
Such database auditing is there to augment business-level auditing, not to replace it. Call centres will sometimes (or always in the case of financial services providers in Australia, as one example) record phone calls. That's part of the audit trail too but doesn't tend to be part of the IT solution as far as the desktop application (and related infrastructure) goes, although that is by no means a hard and fast rule.
Call centre staff will also typically have some sort of "Notes" or "Log" functionality where they can type freeform text as to why the customer called and what action was taken so the next operator can pick up where they left off when the customer rings back.
Triggers will often be used to record exactly what was changed (eg writing the old record to an audit table). The purpose of all this is that with all the information (the notes, recorded call, database audit trail and logs) the previous state of the data can be reconstructed as can the resulting action. This may be to find/resolve bugs in the system or simply as a conflict resolution process with the customer.
It is certainly popular - rails for example has a shorthand for it, as well as a creation timestamp (:timestamps).
At the application level it's very useful, as the same pattern is very common in views - look at the questions here for example (answered 56 secs ago, etc).
It can also be used retrospectively in reporting to generate stats (e.g. what is the growth curve of the number of records in the DB).
there are a couple of scenarios
Let's say you have an address table for your customers
you have your CRM app, the customer calls that his address has changed a month ago, with the LastUpdate column you can see that this row for this customer hasn't been touched in 4 months
usually you use triggers to populate a history table so that you can see all the other history, if you see that the creationdate and updated date are the same there is no point hitting the history table since you won't find anything
you calculate indexes (stock market), you can easily see that it was recalculated just by looking at this column
there are 2 DB servers, by comparing the date column you can find out if all the changes have been replicated or not etc etc ect
This is also very useful if you have to send feeds out to clients that are delta feeds, that is only the records that have been changed or inserted since the data of the last feed are sent.