These examples were the most helpful for me (examples of SF Connect custom adapters for various web services including GitHub, Google Drive, and so on):
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_connector_examples.htm?search_text=github
When the platform calls the DataSource.Connection class's query method, the parameter passed in is the class DataSource.QueryContext. This gives you all of the information of the query context in which you are trying to load external object records (documentation: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_class_DataSource_QueryContext.htm#apex_DataSource_QueryContext_queryMoreToken).
Specifically, the DataSource.QueryContext attribute tableSelection, of type DataSource.TableSelection, contains all of the information you need to structure your callouts (documentation: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_class_DataSource_TableSelection.htm#apex_class_DataSource_TableSelection).
The DataSource.TableSelection class has the properties:
columnsSelected
- List of columns to query. Corresponds to the SELECT clause in a SOQL or SOSL query.
filter
- Identifies the query filter, which can be a compound filter that has a list of subfilters. The filter corresponds to the WHERE clause in a SOQL or SOSL query.
order
- Identifies the order for sorting the query results. Corresponds to the ORDER BY clause in a SOQL or SOSL query.
tableSelected
- Name of the table to query. Corresponds to the FROM clause in a SOQL or SOSL query.
If we take a simple example, where we have some external object Sample__x that we're trying to load, given its indirect lookup to an Account in the external system:
SELECT ExternalId, Account__c, Some_Property__c FROM Sample__x WHERE Account__c = '1234'
Then:
- columnsSelected: [{columnName=ExternalId, ...}, {columnName=Account__c, ...}, {columnName=Some_Property__c, ...}]
- filter: {columnName=Account, columnValue='1234', ...}
- tableSelected: 'Sample__x'
I suggest looking through the documentation in the DataSource namespace, there is quite a good deal of other possibilities and useful classes beyond what the simple example above gives you, including search and query pagination.
