# Advanced Model Building and Customisation

Here we will try and describe usage of some more advanced features that Flux provides to give more control over model building.

## Customising Parameter Collection for a Model

Taking reference from our example Affine layer from the basics.

By default all the fields in the Affine type are collected as its parameters, however, in some cases it may be desired to hold other metadata in our "layers" that may not be needed for training, and are hence supposed to be ignored while the parameters are collected. With Flux, it is possible to mark the fields of our layers that are trainable in two ways.

The first way of achieving this is through overloading the trainable function.

julia> @functor Affine

julia> a = Affine(rand(3,3), rand(3))
Affine{Array{Float64,2},Array{Float64,1}}([0.66722 0.774872 0.249809; 0.843321 0.403843 0.429232; 0.683525 0.662455 0.065297], [0.42394, 0.0170927, 0.544955])

julia> Flux.params(a) # default behavior
Params([[0.66722 0.774872 0.249809; 0.843321 0.403843 0.429232; 0.683525 0.662455 0.065297], [0.42394, 0.0170927, 0.544955]])

julia> Flux.trainable(a::Affine) = (a.W, a.b,)

julia> Flux.params(a)
Params([[0.66722 0.774872 0.249809; 0.843321 0.403843 0.429232; 0.683525 0.662455 0.065297]])

Only the fields returned by trainable will be collected as trainable parameters of the layer when calling Flux.params.

Another way of achieving this is through the @functor macro directly. Here, we can mark the fields we are interested in by grouping them in the second argument:

Flux.@functor Affine (W,)

However, doing this requires the struct to have a corresponding constructor that accepts those parameters.

## Freezing Layer Parameters

When it is desired to not include all the model parameters (for e.g. transfer learning), we can simply not pass in those layers into our call to params.

Consider a simple multi-layer perceptron model where we want to avoid optimising the first two Dense layers. We can obtain this using the slicing features Chain provides:

m = Chain(
Dense(784, 64, relu),
Dense(64, 64, relu),
Dense(32, 10)
)

ps = Flux.params(m[3:end])

The Zygote.Params object ps now holds a reference to only the parameters of the layers passed to it.

During training, the gradients will only be computed for (and applied to) the last Dense layer, therefore only that would have its parameters changed.

Flux.params also takes multiple inputs to make it easy to collect parameters from heterogenous models with a single call. A simple demonstration would be if we wanted to omit optimising the second Dense layer in the previous example. It would look something like this:

Flux.params(m[1], m[3:end])

Sometimes, a more fine-tuned control is needed. We can freeze a specific parameter of a specific layer which already entered a Params object ps, by simply deleting it from ps:

ps = params(m)
delete!(ps, m[2].b)