This is to share some of the tricks you can use while dealing with multi-sites with Sitecore CMS while designing content solutions.

While dealing with multi-site, we all come across scenario where we want data of some fields to be different per site based on where that item exist. In other words, want to load dynamic data in link types of fields for the data present in that local site.

Of course, the answer is use Sitecore query instead of fixed path. The trick I wanted to share is how to formulate that query.

Lets take one example(I feel if you explain with real life example, people are tend to understand more quickly).

My multi-site content information structure is laid out as follow:

Content

SiteA

Home (All the navigable pages goes here)

Configuration (Master settings for site and features)

Data Library (As name suggested this holds data for the site)

SiteB

Home

Configuration

Data Library

Now, lets assume you have SiteA for Asia-Pacific and SiteB for Europe and there are country data items present under Data Library for both respected sites as shown in picture below.

Now, you want content editor to select only countries present under that site in field called “Supported Countries” for site’s Configuration item.

You can have following Sitecore query as Source of field Supported Countries for the Site Configuration template.

query:./ancestor::*[@@templateid='{B8313310-198B-4596-9532-7E39BAB33503}']/Data Library/*[@@templateid='{7CF67E0D-A30F-4C48-9247-69DE07821520}']/*

Let’s dive in details about how this query is formulated. Before doing that, please thoroughly go through image below.

The query is divided in the six parts:

• Part 1: first part is query keyword, which informs Sitecore that the data source of this field is not just simple item path but complex Sitecore query.
• Part 2: The the .(dot) present the current item. These items are highlighted in Red. In our example, it is Configuration item. As we are setting query for the field Supported Countries of template Site Configuration. Configuration item under SiteA and SiteB is actual instance of template.
• Part 3: This is the tricky part, form current item we are getting the list of all the ancestor items. For Configuration item under SiteA, the ancestors are SiteA, Content and Sitecore. For SiteB, ancestors to Configuration items are SiteB, Content and Sitecore. These items are highlighted in violet color. Once we get the list of ancestors, we need to filter out the item which is of template type Site(id of this template is {B8313310-198B-4596-9532-7E39BAB33503}.
• Part 4: From Filtered Site item, we need to dive in the the sub-tree of Library. To do this we can use the same method of filtering based on template id like we did in part 3 above, but this would be bit costly operation as we need to look for all children underneath site and then filter based on Site Data Library template. Here, I have used fixed path instead, assuming all the sites in this content tree have to have a item called “Data Library”. This is to demonstrate, one can use fixed path query as well with Sitecore query. These are highlighted in green color in content tree.
• Part 5: The 5th part is same as part 3, except looking for ancestors, it is looking for children and then filtering the child with the template id {7CF67E0D-A30F-4C48-9247-69DE07821520}. This is to find folder type item of template Country Folder, which are highlighted in Sky-blue color in content tree.
• Part 6: The last part is to get the list of all items under the Countries folder filtered in part 5 above. Here, I assuming none other item present under this folder except for template type Country. If you are looking for particulate type of items you can use filters to get those items. These items in content tree are highlighted in grey color as leaf nodes.

Tip: Be cautious while using descendant or //* in field’s datasource query. This is costlier read operations and may hit the performance of field rendering in Content editor.

Enjoy dynamic field data source and Sitecore query …!!!

Reference
https://sdn.sitecore.net/Reference/Using%20Sitecore%20Query/Sitecore%20Query%20Syntax.aspx