I'm building a web application in yesod with mongodb.
I'm tryign to create a model called Message:
Message
_id Text
threadKey Text
body Text
But I can't seem to access the _id field this way, the message_id function doesn't get created, unlike messageThreadKey and messageBody.
How can I access the _id field of mongo objects from yesod/persistent-mongoDB?
In persistent the id (in both the SQL version and the Mongo version) is special and not actually part of the model record. The combination of a Id and a Model Record is called an Entity.
I would reread the persistent chapter of the Yesod book under the Manipulation section, Insert subsection.
http://www.yesodweb.com/book/persistent
Related
I'm looking at building a system which will include user created fields which will use Postgresql as the database. I've read that the EAV model for this isn't great and that a jsonb column to store the custom fields will be the best route.
I want the user to be able to build their own queries to the data as well so is the best way to go to store the custom field names in a table and store the actual data in the jsonb column, that way when it comes to building the queries the webapp can query the custom fields table for a reference as to what fields are available and their types?
Tickets
ID
WhenCreated
LastUpdated
CustomProperties {JSON B column, key value pairs where the key names will be stored in the custom fields table}
Customfields
ID
FieldName
WhenCreated
FieldTypeID
FieldTypes
ID
FieldTypeName {String, Date, etc}
I have 2 Mongoose.js Schemas that work together with the 'populate' feature: A 'user' schema and another based on their role. E.g. 'admin'. When a user is assigned a role, a corresponding document needs to be created in a different collection with a link to the _id of the document in the users collection. (Yes, more like an SQL database than non-relational, I know)
Currently I manually create the second document in my code whenever a user with a specialized role is created or a role is added to a user. |
I'd like to know if there is a way to automatically create this corresponding record from my schema whenever a 'user' document is created or updated with a role.
Any advice?
Nothing will do it automatically, but you can use the mongoose middleware to insert our update a document in another collection pre or post save.
The post hook will have the _id populated.
If you want to do it in the pre hook (to enforce some transactional integrity) you can assign the _id manually.
I am trying ORMLite as an ORM for a project I am developing. I am mapping a java class to a table that has some auditing fields (ie. updatedby, updatedtime, etc.). The auditing fields are maintained by the database using triggers to ensure that no matter what front-end the user is using these fields will always be correctly updated when a record is updated.
I need to include these fields in my client application to inform the user when the record was last updated, but the user can't change them. Is there a way to annotate the class so that ORMLite won't try to perform updates on these fields or include them in insert statements. The database will deny an update if these fields are included in an update statement (which is why I can't just write back the original value that was queried from the database).
I tried using the #DatabaseField(persisted = false) annotation on the Java fields, but then they don't get queried at all so the Java object is never populated with these fields.
Basically, I need these fields to be included in SELECT statements, but not included in INSERT or UPDATE statements (equivalent to a #DatabaseField(immutable = true) annotation).
Interesting pattern. ORMLite didn't support the feature at the time but now it does as of version 4.46.
There is now a #DatabaseField(readOnly=true) annotation field.
I want to implement content tagging using MongoDB. In a relational database, the best approach would be to have a many-to-many relation between the content (say, "products") and tags tables. But what is best approach with NoSQL databases?
Would it be better to put every tag in a tags array of the "content" document, or put references to tags in a string?
In most cases where you have a n:m relation in MongoDB, you should use embedding instead of referencing. So I would recommend you to have an array "tags" in each product with the tag names. I assume that looking at a single product will be the most frequent use-case in your system. This design will allow you to show the user a product with a list of tag names with a single database query.
When you need some additional meta-data about the tags which you don't want to bind to a product (like a long-text description of a tag), you could create an additional tags collection, where the name field gets an unique index for fast lookup and avoiding duplicates. When the user clicks on or hovers over a tag name, you can use an additional query to get the tag details.
A problematic case in this design is the situation when you want to delete or rename a tag. Then you have to edit every product which includes the tag. But because MongoDB doesn't know foreign keys with CASCADE ON DELETE like SQL databases, you will always have that problem when you have documents referencing one another.
Renaming tags could be made easier by storing objectIDs instead of names in the tag array of the product. But IDs have the disadvantage that they are useless for the user. You need to get the names of the tags to show a product page. That means that you have to request every single one from the tags collection, which requires an additional database query.
I'm just starting out with the Entity Framework and ADO.NET Data Services and I've run into an issue that I can't seem to figure out. I have two tables, one that has user information and the other that has a created by field. Within the database, there isn't a foreign key between these tables. The user table contains an arbitrary Id, a username, and a display name. The created by field contains the user's username. In my entity I would like to have the user's display name since this is what I need to display and expose over the ADO.NET Data Service? I'm aware that I could restructure the database, but I was hoping that I could do the join using the username as I would in a SQL statement.
Thanks in advance,
-Damien
You can make a view using a join of both tables, and then use this object to display the user's name.
There's some info on mapping custom queries here.