Having a list of strings represented in a database

问题:

First of I am new to ORMLite. I would like my model class to have a field which is a list of strings, that would eventually hold a list of tags for my model object. Which ORMLite annotations should I use?

Firstly I don't want to have a table of all tags, and then use the @ForeignCollectionField. Also I thought of using the @DatabaseField(dataType=DataType.SERIALIZABLE) annotation, but it turns out that List<String> doesn't implement the Serializable interface.

What are your suggestions?

回答1:

First of all, List doesn't implement Serializable but ArrayList certainly does as well as most of the common collection implementations. But storing a huge list is probably not the best of doing this from a pure object model standpoint.

So why don't you want to have a table of all tags? That's the best way from a pure model standpoint. It will require a 2nd query if you need them every time. That's the way hibernate would store a list or array of tags.

---

After reading your comment @creen, I still think you do want a table of tags. Your model class would then have:

@ForeignCollectionField
Collection<Tag> tags;

The tags table would not have a single tag named "red" with multiple model classes referring to it but multiple "red" entries. It would look like:

model_id    name
1           "red"
1           "blue"
2           "red"
3           "blue"
3           "black"

Whenever you are removing the model object, you would first do a tags.clear(); which would remove all of the tags associated with that model from the tags table. You would not have to do any extra cleanup or anything.

回答2:

No need to go for @ForeignCollectionField for simple String Array

Change your code

@DatabaseField(dataType=DataType.SERIALIZABLE) 
List<String> users;

to

@DatabaseField(dataType = DataType.SERIALIZABLE)
String[] users;

Database doesn't want to store dynamically grow able arrays. That is the reason it allows only static array like string[] and not List.

回答3:

I added two properties... one that gets written to the database as a csv string and the other that translates this:

[Ignore]
public List<string> Roles
{
    get
    {
        return new List<string>(RolesCsv.Split(new char[] { ',' }));
    }
    set
    {
        RolesCsv = string.Join(",", value);
    }
}
public string RolesCsv { get; set; }