Mapping document indexing with NEST

I'm using .NET Core application with NEST client.
My goal is to index a model with few modifications in its properties, for example, this model contains list of objects, and I would like to document list of the Ids of the objects instead of the objects themselves.

So, I have made a test, I created a mapping, according to the information I have found over here:

var result = elasticClient.Indices.Create("change-log", c => c
    .Map<ChangeLog>(m => m
        .Properties(ps => ps
            .Text(s => s
                .Name(n => n.Source)
            .Text(s => s

After this code runs, I can see in Index Management - Mapping that my mapping was just created for 'change-log' index.

Right after the execution of the previous code I'm trying to index my model with the following code:

var indexResponse1 = elasticClient.Index(data, i => i.Index("change-log"));

The model inside data variable indexed regardless the mapping that was just created, and when I am looking again in Index Management - Mapping I can see that mapping was changed to the structure of the model held inside data variable.

What is wrong with my code? would be grateful for your help.