Help getting started with PetaPoco
I’ve recently spent some time working with PetaPoco. As always with something new I found problems early and often but not real problems with PetaPoco itself, just typical “What does this error mean?” and “How do I do this then?”
The kind of objects I want to use with PetaPoco are domain entities so there’s a bit more meat to them than a flat poco. Here are the test classes I started with :
public class Author
{
public int id {get; internal set;}
public string name {get; internal set;}
}
public class Article
{
public int id {get; internal set;}
public string content {get; internal set;}
public string title {get; internal set;}
public Author author {get; internal set;}
}
The property setters are internal so that client (application) code using the entities cannot change their values. The only way these properties would be changed is either :
- in the repository when the entities is being loaded (hydrated)
- during construction or through a builder class
- through public methods on the entities
In my real code I’ll use the InternalsVisibleTo attribute on the classes to ensure that outside of the domain classes only the repository assembly can access the setters. This test is stripped right back so the only remnant of these requirements are the internal setters. Also notice that there is a relationship between Article and Author; when I load an Article I want the Author to be brought back too. So now here are the learning curve issues I had :
Querying data
Let’s load a list of articles from the database and join the author table too :
db.Query(
"SELECT * FROM article left outer join " +
"author on author.id = article.author_id")
This throws an exception :
Unhandled Exception: System.ArgumentNullException: Value cannot be null. Parameter name: meth at System.Reflection.Emit.DynamicILGenerator.Emit(OpCode opcode, MethodInfo meth)
This happens because PetaPoco currently only looks for public setters when mapping to pocos and all my setters are internal. This is easily fixed in PetaPoco.cs by changing all instances of GetSetMethod() to GetSetMethod( true) (update: this change was added to PetaPoco core shortly after this post, so you won’t need to apply it)
Now I want to load a single article. Let’s try using the new multi-poco support :
var article = db.Single(
"SELECT * FROM article left outer join author" +
" on author.id = article.author_id");
This doesn’t compile because the multi-poco feature only applies to the Query() and Fetch() functions at the moment so I can only pass in the type of the result which is Article. The solution is simple :
var article = db.Query(
"SELECT * FROM article left outer join author " +
" on author.id = article.author_id where article.id=@0",
1).SingleOrDefault();
Inserting data
Ok let’s try and insert a new article. We’ll leave out the author for now and see if the foreign key constraint in the database kicks in.
int newRecordId = (int)db.Insert( new Article() {title="Article 1",
content="Hello from article 1"});
PetaPoco didn’t like it :
Unhandled Exception: Npgsql.NpgsqlException: ERROR: 42P01: relation “Article” does not exist at Npgsql.NpgsqlState.d__a.MoveNext()
I’m using PostgreSQL and it seems that it is case sensitive, if I change my class names to all lower case it works. But I don’t want to do that so all I need to do is use the TableName attribute which overrides the convention of class name equals table name :
[TableName( "author")]
public class Author
{
public int id {get; internal set;}
public string name {get; internal set;}
}
[TableName( "article")]
public class Article
{
public int id {get; internal set;}
public string content {get; internal set;}
public string title {get; internal set;}
public Author author {get; internal set;}
}
Now I can get a bit further but I still get an error :
Unhandled Exception: Npgsql.NpgsqlException: ERROR: 42703: column “author” of relation “article” does not exist at Npgsql.NpgsqlState.d__a.MoveNext()
This happens because my poco has an author property and this doesn’t map directly to the author_id column on the database. The author_id column is an integer, so how can I pass my poco into Update() and have it use an author id? You can’t. Instead you need to use a different overload of the Insert() method :
int newRecordId = (int)db.Insert( "article", "id", true,
new {title="A new article",
content="Hello from the new article",
author_id=(int)1});
We are now no longer using the poco, we are using an anonymous type. This is doubly useful because we now have the option to just update specific columns if we want to. Code calling my repository will still pass in a poco but the repository will strip out the values it wants to update. I like that.
In this example I already knew the author id because the user will have selected the author from a drop down while entering the article details.
Updating data
Now let’s update an existing article :
var newArticle = db.Query(
"SELECT * FROM article left outer join author on" +
"author.id = article.author_id where article.id=@0",
1).SingleOrDefault();
newArticle.content += " The end.";
db.Update( newArticle);
Here is this code’s exception :
Unhandled Exception: System.InvalidCastException: Can’t cast PetaPoco_MultiPoco_
Test.Author into any valid DbType.
at Npgsql.NpgsqlParameter.set_Value(Object value)
Again, PetaPoco can’t map the author property to a column on the article table. The way around this is the tell PetaPoco that the author property shouldn’t be used during inserts or updates. We use the ResultColumn attribute for this :
public class Article
{
public int id {get; internal set;}
public string content {get; internal set;}
public string title {get; internal set;}
[ResultColumn]
public Author author {get; internal set;}
}
Now we get another error, remember that I’m using PostgreSQL here :
Unhandled Exception: Npgsql.NpgsqlException: ERROR: 42703: column “ID” does not exist at Npgsql.NpgsqlState.
d__a.MoveNext()
This happens because PetaPoco will use “ID” as the default primary key if you don’t specify it. There are three solutions to this. Firstly I could change my database to use “ID” as the column name. Secondly I could change PetaPoco.cs to use “id” as the default. This would be done in the constructor for the PocoData internal class :
TableInfo.PrimaryKey = a.Length == 0 ? "id" : (a[0] as PrimaryKeyAttribute).Value;
Thirdly, and preferably, I’ll use the PrimaryKey attribute on my poco classes :
[TableName( "author")]
[PrimaryKey("id")]
public class Author
{
public int id {get; internal set;}
public string name {get; internal set;}
}
[TableName( "article")]
[PrimaryKey("id")]
public class Article
{
public int id {get; internal set;}
public string content {get; internal set;}
public string title {get; internal set;}
[ResultColumn]
public Author author {get; internal set;}
}
Also note that if we want to update the author we’d just do a separate call to Update(). If I find anything else worth sharing I’ll update this post