Nested Mail Merge with Regions
In some scenarios, you may need to use nested mail merge with regions. Nested merge is a feature that enables you to merge hierarchical data from your data source into your merge template to easily populate your document. Basically, the hierarchical data is represented as a set of data items, and hierarchical relationships describe how the data items are related to each other (one item of data is the parent of another one).
Aspose.Words allows you to perform a mail merge operation with nested regions. You can use this feature if you have a data source that is organized into a tree-like structure and you want to execute a mail merge operation to populate a template with hierarchical data.
What is a Nested Mail Merge
The mail merge region is called nested if you have two or more mail merge regions where one of them is inside the other in a hierarchical form. Note that each region contains data from one table.
The most common example of a nested mail merge is an order that contains multiple items where you need to link multiple data tables and present the information in a template.
The picture below shows two nested regions where the Order mail merge region is the parent of the Item mail merge region.
How to Process Mail Merge with Nested Regions
The data to be merged into a template can come from various sources, mainly relational databases or XML documents. In our example, we are going to use an XML file to store our data and load it directly into the DataSet.
Aspose.Words allows you to process mail merge with nested regions using the data relationships specified in the DataSet. When the DataSet object loads XML, it either uses the provided schema or infers it from the structure of the XML itself to accomplish this. From this structure, it creates relationships between tables where necessary.
The image below shows how the data from the Order table passed to the nested merge regions will be linked to the Item table, as well as the output generated during the merge operation.
As you can see from the output document, each order from the Order table is inserted into the merge template with all order’s related items from the Item table. The next order will be inserted along with their items until all the orders and items are listed. The order of nesting mail merge with regions in the template must match the data relationships between the tables in the data source.
The following code example shows how to generate an invoice using nested mail merge with regions:
How to Set Up Data Relations in Nested Mail Merge with Regions
You need to set up all data relationships in the parent-child structure to execute the nested mail merge with regions correctly. Skipping this important step can lead to a failure in executing the nested mail merge with regions.
When retrieving data for a nested mail merge from an XML file using the ReadXml method, relationships are automatically created according to the structure of the XML document. However, you need to make sure that correct relations have been created.
If mail merge is not working as expected, then you may need to restructure your XML file or explicitly create relations between DataTable objects in the DataSet.
A DataSet that has related data tables will use the DataRelation object to represent the parent-child relationship between the tables.
The following code example shows how to establish a DataRelation between a customer’s table and an order’s table by using a DataRelation object:
dataSet.getRelations().add(new DataRelation("OrderToItem", orderTable.getColumns().get("Order_Id"), itemTable.getColumns().get("Order_Id"), false));
Common Nested Mail Merge Problems and Solutions
The table below provides typical problems with performing nested mail merge via Aspose.Words, as well as their respective solutions.
|The generated output has no fields that are merged.||The original name of the merge field stays the same without being replaced by the required data from the data source.||- Check the data is being loaded properly into tables: set the TableName correctly with all required primary keys and relationships.
- Check that the merge fields are named properly. Use the GetFieldNames method to get all merge fields names and ensure that the name of merge fields in your template matches the one in your data source.
|The output of nested merging displays no data from the child table for the first entry in the parent table but displays all items for the last entry in the parent table, even ones that are not actually linked to it.||The merge regions in the template are not correctly formed that can make all nested mail merge regions to stop displaying anything at all.||The StartTable opening tag and the EndTable closing tag must match and be in the same row or cell. For example, if you start nesting merge regions in a cell of a table, you must end the merge region in the same row as the first cell.|
|Each entry from the parent table displays every item in the child table, even ones that are not actually linked to it.||The relationship between the parent and child tables are not set up or they are incorrectly set up.||- Ensure data integrity within your DataSet and use the DataRelatoin object to represent the parent-child relationship between related data tables.
- Check the section explained above in this article on “How to Set Up Data Relations in Nested Mail Merge with Regions”.
|An exception is thrown while executing nested mail merge as the following: “System.ArgumentException: This constraint cannot be enabled as not all values have corresponding parent values”.||Not every parent record has a child record so your data source does not match the following criteria:Every row in the parent table should have a one-to-one relationship with the rows of the child table based on the primary and foreign keys.||Disable foreign key constraints when you create a DataRelation.|