First let me say if you’re reading this and you haven’t checked out Breeze.js, you should.  It’s a nice little library.  I’m using it in an Angular.js based application and it has greatly simplified data access.  Now on to the show.

I’m using Breeze.js for data access and as part of that I am registering some constructors like so.

var AddressCtor = function () {
    this.CompanyId = document.getElementById('CompanyId').value;
};

var NoteCtor = function () {
    this.CompanyId = document.getElementById('CompanyId').value;
};

metadataStore.registerEntityTypeCtor('Address', AddressCtor);
metadataStore.registerEntityTypeCtor('Note', NoteCtor);

Everything was working fine then I tried to do a little code cleanup.  The constructors for Note and Address were identical.   So, of course I, re-factored them into a single generically named function.

var CompanyEntity = function () {
    this.CompanyId = document.getElementById('CompanyId').value;
};

metadataStore.registerEntityTypeCtor('Address', CompanyEntity);
metadataStore.registerEntityTypeCtor('Note', CompanyEntity);

Well, it appears that you cannot register 2 constructors that call the same function like that.  If you do you the error “The Key is Already Attached: Note:Webapplication1″, and no data loaded in the Angular app.

When I finally found the cause, I couldn’t believe it.  The fix that I came up with was to pass null as the constructor parameter and move that CompanyId assignment into a function that is passed as the init parameter.  This has the added bonus that entities that need to do a series of things AND set the CompanyId can just call the companyEntityInit function from inside their initializers.

function companyEntityInit(entity) {
    entity.CompanyId = document.getElementById('CompanyId').value;
};

metadataStore.registerEntityTypeCtor('Address', null, companyEntityInit);
metadataStore.registerEntityTypeCtor('Note', null, companyEntityInit);