I've made a guestbook application using Google App Engine(GAE):python and the client is running on iPhone.
It has ability to write messages on the board with nickname.
The entity has 3 fileds:
nickname
date
message
And I'm about to make another feature that user can post reply(or comment) on a message.
But to do this, I think there should a 'primary key' to the guestbook entity, so I can put some information about the reply on a message.
With that three fields, I can't get just one message out of database.
I'm a newbie to database. Does database save some kind of index automatically? or is it has to be done by user?
And if it's done automatically by database itself(or not), how can I get just one entity with the key??
And I want to get some advise about how to make reply feature generally also. Thanks to read.
Every entity has a key. If you don't assign a key_name when you create the entity, part of the key is an automatically-assigned numeric ID. Properties other than long text fields are automatically indexed unless you specify otherwise.
To get an entity if you know the key, you simply do db.get(key). For the replies, you probably want to use a db.ReferenceProperty in the reply entity to point to the parent message; this will automatically create a backreference query in the message to get replies.
Each entity has a key, it contains information such as the kind of entity it is, it's namespace, parent entities, and the most importantly a unique identifier (optionally user specifiable).
You can get the key of an entity using the key method that all entities have.
message.key()
A key can be converted to and from a URL-safe string.
message_key = str(message.key())
message = Message.get(message_key)
If the key has a user-specified unique identifier (key name), you can access it like this
message.key().name()
Alternatively, if a key name was not specified, an id will be automatically assigned.
message.key().id()
To assign a key name to an entity, you must specify it when creating the entity, you are not able to add/remove or change the key name afterwards.
message = Message(key_name='someusefulstring', content='etc')
message.put()
You will then be able to fetch the message from the datastore using the key name
message = Message.get_by_key_name('someusefulstring')
Use the db.ReferenceProperty to store a reference to another entity (can be of any kind)
It's a good idea to use key name whenever possible, as fetching from the datastore is much faster using them, as it doesn't involve querying.
Related
I want to know the best way of avoiding entity duplication in an ASP.NET Core Web API project.
Imagine that you have a product with a name and manufacturer and you want to make sure if the name is not duplicated. Imagine that a new product with a name came from client (dto) and we need to look if the name (entity) already exists in the database (using EF).
You need to trim the name (name.trim()) for both names from entity and dto
You need to remove all the whitespaces in between (string.replace(" ", string.empty())
You need to change everything to lower case (string.lower())
Finally you need to compare these two
Is there any best practices how to do this without writing all the code? I tried to use string.compare with the compareoptions like ignorecase and ignoresymbols and also the string.equal() with ignorecase option but the EF gives me an alarm that it can not translate the code.
br
I have a suggestion for your approach.
Introduce another column (This can be a primary key with other keys) and save the name with trimming and lowercase when you insert a new record to that table.
Example:
Original Name : Amir Masoud Babaei -->
New Column value: amirmasoudbabaei
And when you insert a new record, do your trimming and lowercase changes and save it to the database. Since it is a primary key, it should throw an error.
So with this approach, you don't need to loop through all the names and validate if the name is already exist.
An Entity Id is required for each new entity in the broker.
In NGSI-LD the format for the Entity Id is urn:ngsi-ld:TemperatureSensor:001.
Is it possible to somehow autoincrement the number of the Entity so that the new Entity can be created programmatically?
For example, provide something like urn:ngsi-ld:TemperatureSensor:* and the new Entity can be created with an auto-incremented Entity Id.
Thanks.
The id should be generated by the application itself. You have multiple ways of generating unique ids but I would recommend ids that have some semantics behind so that you can, for instance, infer from an id in which area the sensor is located, who the owner is, etc.
I have an entity group in objectify, typical SomeParentClass and SomeChildClass. I want to do something like this to load an instance of SomeChildClass from the datastore.
ofy().load.type(SomeChildClass.class).id(idOfSomeChildClassInstace);
This is returning nothing found. Seems that you need to know the parent of SomeChildClass to get it from the datestore. This I know works.
Key<SomeChildClass> k = Key.create(someParentClass.generateKey(), SomeChildClass.class, idOfSomeChildClassInstace);
ofy().load().key(k).now;
What if I want to load an instance of SomeChildClass without knowing the parent, by just having the id of SomeChildClass.
You cannot do that - the actual full identifier of an entity is the kind and id of each of its ancestors as well as it's own kind and id. That is why building the full key works, but using just the child entity id does not. Another way of looking at it that ids are only unique between siblings of the same parent.
The easiest way to solve your issue is to produce a key for your child entity, then get the 'web safe string' for it. This string contains all the information of the entity and all it's parents and can be used to fully reconstitute the full id.
Using objectify:
String websafeKey = Key.create(parentKey, Entity.class, id).getString();
Key<Entity> key = Key.create(websafeKey);
You can also do this with the low level api if you need to.
You need to know the whole Key to be able to get() an entity. A child key consists of: kind, ID and parent key. So you need to provide all three.
Is there a way to force the ID value for a new entity in EF when we have an auto-incrementing ID column, i.e. use SET IDENTITY_INSERT behaviour through EF?
Our requirement is that our create form must always show a new, unique ID for the object we're creating on the empty form before it is filled in or saved. The idea is that this ID can be out read to someone over the phone and then the user can complete and save the form after the call is complete. We could reserve an ID by inserting an empty row into the database there and then, but we have unique columns and FKs; instead I've made a 'next ID' table that we increment with locks for safety, and I test this against the top ID in the object table too to be careful. The idea was to then force the use of this new ID when we write back the entity - but I can't see how to get EF to do it.
Is that possible - is it just something I've missed? I don't think the ID even makes it down to the insert so I don't think manually calling SET IDENTITY_INSERT around the SaveChanges would help.
Or do I have to do something else? I can see alternatives:
Change our ID column to not be an identity and take manual control of it all: there's a table ID inheritance here so this is potentially tricky too.
Separate DB ID and user-visible ID into a separate column, and record our unique ID there.
Empty row to reserve the ID, as above; might need some nullability changes, and amending our data read code to ignore these records.
Thanks! This is EF4 (using an EDMX and generated classes not POCOs), and against SQL Server 2008 in case that matters.
Why not use a Guid as primary key. Nothing to do with auto-increment, no concurrency pitfalls etc. You just create the Guid at the moment you create the form. Hand it over to a caller and fill in the form afterwards. When the form is cancelled, no problem. When the form is finished create the entity with the created Guid set the other values of the entity object, apply it to the (a) context and SaveChanges()...
Alternatives that wont alter your schema
Use EF Transaction
You can call context.SaveChanges() and get the autoincremented primary key. Once the process is completed you can commit the transaction. If the transaction is cancelled or there is an error/exception, you can always rollback so you wont have holes/dirty-data in your rows. I suggest you use the singleton pattern and pass the same transaction/context to whatever methods or screens to complete the process.
Just add an additional status: Draft
Save empty form as draft with saved ID, then proceed to edit the form with the information. Once complete save the form as final/ready. If you wont proceed to save the form, you can always recycle the draft.
I'm getting more and more frustrated with EF...
I have a table called ExtendedField with:
Record
DocRef
DocType
Name
Record is the primary key
DocRef and DocType are foreign keys used to identify which Ticket they belong to
Name is the key used by the "definition" table to define what the field actually is
So basically I need 2 associations:
One between Ticket and ExtendedField
on ExtendedField.DocRef=ticket.record
and
ExtendedField.docType=HeaderDocType
One between Definition on
ExtendedField.Name=Definition.FieldName
Then I still need Record to be the primary key so I can directly access the fields.
As near as I can tell this is impossible to do in Entity Framework. For every association all the keys need to be mapped together, whereas I need two keys for one association, 1 key for another one and the actual primary key wouldn't be used in any associations.
It doesn't appear that you can define an association between fields that aren't entity keys either.
So is there any way to do this? Am I missing something?
It's a v1, bro. I myself have had some major pain with mapping of key constraints in EF. I hear that better things are coming in v2.