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To learn more about object constraint language expressions (OCL expressions) you can watch the OCL Sessions Series
We can use the system protoyper to connect to our database and write expressions. There are objects in the database that we need to transport into our application. And this is exactly what the turnkey application does by using different expressions.
One common operator is all instances. It takes all of the class referenced and post them back to the application. Following the explanation in the video if we have a set of objects, we have different types of operators - likes size, for example, it would return 6, but in our case, we have 4 cars. So we can stack the operators after each other and use all instances to make size returns 4.
There are many different operators, one of them is "last". It checks the list of retrieved or selected objects and picks the last one. There's an operator called "first" - that does almost the same thing, but with the first one.
Order by is an operator that could sort our objects on some property. Let's say that is B C A D E F "order by" on that property would fetch "A" first "B" then "C" "D" "E" "F".
SubSequence operator takes only a subset of returned set from allinstances. For example 1,2, like that, so instead of 4 we have got only two and we could combine. So following the video, that's add the order by registration number behind the subsequence expression that we have in the debugger.
The most important operator that you will use most likely is 'select' it requires some boolean value, so it checks each and every value object that runs through on some property. Then it either chooses to return it or not. When it comes to select you can combine any boolean expression like logic operators with 'and' and 'or' 'not' and things like that. One important concept in using select is the loop variable - it is the name on the iterated object in the expressions. For example, we are going to call that "one car". There's a pipe sign, it means that we could use this variable "one car" in our expressions. It helps when the expressions get complex.